Switchboard Assembly

The a embly of all the parts of a simple magneto switchboard into a complete whole deserves final consideration. The structure in which the various parts are mounted, referred to as the cabinet, is usually of wood. Functions of Cabinet. The purpose of the cabinet is not only to form a support for the various pieces of apparatus but also to protect them from dust and mechanical injury, and to hold those parts that must be manipulated by the operator in such relation that they may be most convenient for use, and thus best adapted for carrying out their various functions. Other points to be provided for in the design of the cabinet and the arrangement of the various parts within are: that all the apparatus that is in any way liable to get out of order may be readily acce ible for inspection and repairs; and that provision shall be made whereby the wiring of these various pieces of apparatus may be done in a systematic and simple way so as to minimize the danger of cro ed, grounded, or open circuits, and so as to provide for ready repair in case any of these injuries do occur. Wall-Type Switchboards. The simplest form of switchboard is that for serving small communities in rural districts. Ordinarily the telephone industry in such a community begins by a group of farmers along a certain road building a line connecting the houses of several of them and installing their own instruments. This line is liable to be extended to some store at the village or settlement, thus affording communication between these farmers and the center of their community. Later on those residing on other roads do the same thing and connect their lines to the same store or central point. Then it is that some form of switchboard is established, and perhaps the storekeeper's daughter or wife is paid a small fee for attendance. IMG:384091999325875926_fig292_t.gif.png:Illustration_ Fig. 292. Wall Switchboard with Telephone Fig. 292. Wall Switchboard with Telephone View full size illustration. A switchboard well-adapted for this cla of service where the number of lines is small, is shown in Fig. 292. In this the operator's talking apparatus and her calling apparatus are embodied in an ordinary magneto wall telephone. The switchboard proper is mounted alongside of this, and the two line binding posts of the telephone are connected by a pair of wires to terminals of the operator's plug, which plug is shown hanging from the left-hand portion of the switchboard. The various lines centering at this point terminate in the combined drops and jacks on the switchboard, of which there are 20 shown in this illustration. Beside the operator's plug there are a number of pairs of plugs shown hanging from the switchboard cabinet. These are connected straight through in pairs, there being no clearing-out drops or keys a ociated with them in the arrangement. Each line shown is provided with an extra jack, the purpose of which will be presently understood. The method of operation is as follows: When a subscriber on a certain line desires to get connection through the switchboard he turns his generator and throws the drop. The operator in order to communicate with him inserts the plug in which her telephone terminates into the jack, and removes her receiver from its hook. Having learned that it is for a certain subscriber on another line, she withdraws her plug from the jack of the calling line and inserts it into the jack of the called line, then, hanging up her receiver, she turns the generator crank in accordance with the proper code to call that subscriber. When that subscriber responds she connects the two lines by inserting the two plugs of a pair into their respective jacks, and the subscribers are thus placed in communication. The extra jack a ociated with each line is merely an open jack having its terminals connected respectively with the two sides of the line. Whenever an operator desires to listen in on two connected lines she does so by inserting the operator's plug into one of these extra jacks of the connected lines, and she may thus find out whether the subscribers are through talking or whether either one of them desires another connection. The drops in such switchboards are commonly high wound and left permanently bridged acro the line so as to serve as clearing-out drops. The usual night-alarm attachment is provided, the buzzer being shown at the upper right-hand portion of the cabinet. IMG:384091999325875926_fig293_t.gif.png:Illustration_ Fig. 293. Combined Telephone and Switchboard Fig. 293. Combined Telephone and Switchboard View full size illustration. Another type of switchboard commonly employed for this kind of service is shown in Fig. 293, in which the telephone and the switchboard cabinet are combined. The operation of this board is practically the same as that of Fig. 292, although it has manually-restored drops instead of self-restoring drops; the difference between these two types, however, is not material for this cla of service. For such work the operator has ample time to attend to the restoring of the drop and the only po ible advantage in the combined drop-and-jack for this cla of work is that it prevents the operator from forgetting to restore the drops. However, she is not likely to do this with the night-alarm circuit in operation, since the buzzer or bell would continue to ring as long as the drop was down. IMG:384091999325875926_fig294_t.gif.png:Illustration_ Fig. 294. Upright Magneto Switchboard Fig. 294. Upright Magneto Switchboard View full size illustration. IMG:384091999325875926_fig295_t.gif.png:Illustration_ Fig. 295. Upright Magneto Switchboard—Rear View Fig. 295. Upright Magneto Switchboard—Rear View View full size illustration. Upright Type Switchboard. By far the most common type of magneto switchboard is the so-called upright type, wherein the drops and jacks are mounted on the face of upright panels rising from a horizontal shelf, which shelf contains the plugs, the keys, and any other apparatus which the operator must manipulate. Front and rear views of such a switchboard, as manufactured by the Kellogg Company, are shown in Figs. 294 and 295. This particular board is provided with fifty combined drops and jacks and, therefore, equipped for fifty subscribers' lines. The drops and jacks are mounted in strips of five, and arranged in two panels. The clearing-out drops, of which there are ten, are arranged at the bottom of the two panels in a single row and may be seen immediately above the switchboard plugs. There are ten pairs of cords and plugs with their a ociated ringing and listening keys, the plugs being mounted on the rear portion of the shelf, while the ringing and listening keys are mounted on the hinged portion of the shelf in front of the plugs. IMG:384091999325875926_fig296_t.gif.png:Illustration_ Fig. 296. Details of Drop, Jack, Plug, and Key Arrangement Fig. 296. Details of Drop, Jack, Plug, and Key Arrangement View full size illustration. IMG:384091999325875926_fig297_t.gif.png:Illustration_ Fig. 297. Cro -Section of Upright Switchboard (partial) Fig. 297. Cro -Section of Upright Switchboard (partial) View full size illustration. A better idea of the arrangement of drops, jacks, plugs, and keys may be had from an illustration of a Dean magneto switchboard shown in Fig. 296. The clearing-out drops and the arrangement of the plugs and keys are clearly shown. The portion of the switchboard on which the plugs are mounted is always immovable, the plugs being provided with seats through which holes are bored of sufficient size to permit the switchboard cord to pa beneath the shelf. When one of these plugs is raised, the cord is pulled up through this hole thus allowing the plug to be placed in any of the jacks. The key arrangement shown in this particular cut is instructive. It will be noticed that the right-hand five pairs of plugs are provided with ordinary ringing and listening keys, while the left-hand five are provided with party-line ringing keys and listening keys. The listening key in each case is the one in the rear and is alike for all of the cord pairs. The right-hand five ringing keys are so arranged that pre ing the lever to the rear will ring on the answering cord, while pre ing it toward the front will cause ringing current to flow on the calling plug. In the left-hand five pairs of cords shown in this cut, the pre ure of any one of the keys causes a ringing current of a certain frequency to flow on the calling cord, this frequency depending upon which one of the keys is pre ed. IMG:384091999325875926_fig298_t.gif.png:Illustration_ Fig. 298. Cord Weight Fig. 298. Cord Weight View full size illustration. An excellent idea of the grouping of the various pieces of apparatus in a complete simple magneto switchboard may be had from Fig. 297. While the arrangement here shown is applicable particularly to the apparatus of the Dean Electric Company, the structure indicated is none-the-le generally instructive, since it represents good practice in this respect. In this drawing the stationary plug shelf with the plug seat is clearly shown and also the hinged key shelf. The hinge of the key shelf is an important feature and is universally found in all switchboards of this general type. The key shelf may be raised and thus expose all of the wiring leading to the keys, as well as the various contacts of the keys themselves, to inspection. IMG:384091999325875926_fig299_t.gif.png:Illustration_ Fig. 299. Magneto Switchboard, Target Signals (partial) Fig. 299. Magneto Switchboard, Target Signals (partial) View full size illustration. As will be seen, the switchboard cords leading from the plugs extend down to a point near the bottom of the cabinet where they pa through pulley weights and then up to a stationary cord rack. On this cord rack are provided terminals for the various conductors in the cord, and it is at this point that the cord conductors join the other wires leading to the other portions of the apparatus as required. A good form of cord weight is shown in Fig. 298; and obviously the function of these weights is to keep the cords taut at all times and to prevent their tangling. IMG:384091999325875926_fig300_t.gif.png:Illustration_ Fig. 300. Rear View of Target Signal, Magneto Switchboard Fig. 300. Rear View of Target Signal, Magneto Switchboard View full size illustration. The drawing, Fig. 297, also gives a good idea of the method of mounting the hand generator that is ordinarily employed with such magneto switchboards. The shaft of the generator is merely continued out to the front of the key shelf where the usual crank is provided, by means of which the operator is able to generate the nece ary ringing current. Beside the hand generator at each operator's position, it is quite common in magneto boards, of other than the smallest sizes, to employ some form of ringing generator, either a power-driven generator or a pole changer driven by battery current for furnishing ringing current without effort on the part of the operator. IMG:384091999325875926_fig301_t.gif.png:Illustration_ Fig. 301. Dean Two-Position Switchboard Fig. 301. Dean Two-Position Switchboard View full size illustration. Switchboards as shown in Figs. 294 and 295, are called single-position switchboards because they afford room for a single operator. Ordinarily for this cla of work a single operator may handle from one to two hundred lines, although of course this depends on the amount of traffic on the line, and this, in turn, depends on the character of the subscribers served, and also on the average number of stations on a line. Another single-position switchboard is shown in Figs. 299 and 300, being a front and rear view of the simple magneto switchboard of the Western Electric Company, which is provided with the target signals of that company rather than the usual form of drop. Where a switchboard must accommodate more lines than can be handled by a single operator, the cabinet is made wider so as to afford room for more than one operator to be seated before it. Sometimes this is accomplished by building the cabinet wider, or by putting two such switchboard sections as are shown in Figs. 294 or 299 side by side. A two-position switchboard section is shown in front and rear views in Figs. 301 and 302. IMG:384091999325875926_fig302_t.gif.png:Illustration_ Fig. 302. Rear View of Dean Two-Position Switchboard Fig. 302. Rear View of Dean Two-Position Switchboard View full size illustration. Sectional Switchboards. The problem of providing for growth in a switchboard is very much the same as that which confronts one in buying a bookcase for his library. The Western Electric Company has met this problem, for very small rural exchanges, in much the same way that the sectional bookcase manufacturers have provided for the po ible increase in bookcase capacity. Like the sectional bookcase, this sectional switchboard may start with the smallest of equipment—a single sectional unit—and may be added to vertically as the requirements increase, the original equipment being usable in its more extended surroundings. IMG:384091999325875926_fig303_t.gif.png:Illustration_ Fig. 303. Sectional Switchboard—Wall Type Fig. 303. Sectional Switchboard—Wall Type View full size illustration. This line of switchboards is illustrated in Figs. 303 to 306. The beginning may be made with either a wall type or an upright type of switchboard, the former being mounted on brackets secured to the wall, and the latter on a table. A good idea of the wall type is shown in Fig. 303. Three different kinds of sectional units are involved in this: first, the unit which includes the cords, plugs, clearing-out drops, listening jacks, operator's telephone set and generator; second, the unit containing the line equipment, including a strip of ten magneto line signals and their corresponding jacks; third, the finishing top, which includes no equipment except the support for the operator's talking apparatus. IMG:384091999325875926_fig304_t.gif.png:Illustration_ Fig. 304. Sectional Switchboard—Wall Type Fig. 304. Sectional Switchboard—Wall Type View full size illustration. The first of the units in Fig. 303 forms the foundation on which the others are built. Two of the line-equipment units are shown; these provide for a total of twenty lines. The top rests on the upper line-equipment unit, and when it becomes nece ary to add one or more line-equipment units as the switchboard grows, this top is merely taken off, the other line-equipment units put in place on top of those already existing, and the top replaced. The wall type of sectional switchboard is so arranged that the entire structure may be swung out from the wall, as indicated in Fig. 304, exposing all of the apparatus and wiring for inspection. Each of the sectional units is provided with a separate door, as indicated, so that the rear door equipment is added to automatically as the sections are added. In the embodiment of the sectional switchboard idea shown in these two figures just referred to, no ringing and listening keys are provided, but the operator's telephone and generator terminate in a special plug—the left-hand one shown in Fig. 303—and when the operator desires to converse with the connected subscribers, she does so by inserting the operator's plug into one of the jacks immediately below the clearing-out drop corresponding to the pair of plugs used in making the connection. The arrangement in this case is exactly the same in principle as that described in Fig. 292. The operator's generator is so arranged in connection with this left-hand operator's plug that the turning of the generator crank automatically switches the operator's telephone set off and switches the generator on, just the same as a switch hook may do in a subscriber's series telephone. IMG:384091999325875926_fig305_t.gif.png:Illustration_ Fig. 305. Sectional Switchboard—Table Type Fig. 305. Sectional Switchboard—Table Type View full size illustration. IMG:384091999325875926_fig306_t.gif.png:Illustration_ Fig. 306. Sectional Switchboard—Table Type Fig. 306. Sectional Switchboard—Table Type View full size illustration. The upright type of sectional switchboard is shown in Figs. 305 and 306, which need no explanation in view of the foregoing, except to say that, in the particular instrument illustrated, ringing and listening keys are provided instead of the jack-and-plug arrangement of the wall type. In this case also, the top section carries an arm for supporting a swinging transmitter instead of the hook support for the combined transmitter and receiver. ToC rEVIEW QUESTIONS REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 11—62   1. When was the telephone invented and by whom?   2. State the velocity of sound in air. Is it higher in air than in a denser medium?   3. State and define the characteristics of sound.   4. Make sketch of Bell's original magneto telephone without permanent magnets.   5. Describe and sketch Hughes' microphone.   6. Which is, at present, the best material for varying the resistance in transmitters?   7. Give the fundamental differences between the magneto transmitter and the carbon transmitter.   8. What is the function of the induction coil in the telephone circuit?   9. Describe and sketch the different kinds of visible signals. 10. What should be the diameter of hard drawn copper wire in order to allow economical spacing of poles? 11. State the four principal properties of a telephone line. 12. If in testing a line the capacity is changed what are the results found on the receiver and transmitter end? 13. Why is paper used as an insulator of telephone cables? 14. How does a conductor behave in connection with direct current and how with alternating current? 15. What influence has inductance on the telephone? 16. Define impedance and give the formula for it. 17. What is the usual specification for insulation of resistance in telephone cables? 18. If 750 feet of cable have an insulation resistance of 9,135 megohms, how great is the insulation resistance for 7 miles and 1,744 feet of cable? 19. What is the practical limiting conversation distance for No. 10 B. and S. wire? 20. Describe Profe or Pupin's method of inserting inductance into the telephone line. 21. What does mho denote? 22. Why are Pupin's coils not so succe ful on open wires? 23. What is a repeater? 24. Define reactive interference . 25. State the frequencies of the pitches of the human voice. 26. What is the office of a diaphragm in a telephone apparatus? 27. What transmitter material has greatly increased the ranges of speech? 28. Describe the different methods of measurements of telephone circuits. 29. What are the two kinds of electric calls ? 30. How many conductors has a telephone line? 31. Give formula for capacity reactance and the meaning of the symbols. 32. Which American cities are joined by underground lines at present? 33. State the two practical ways of improving telephone transmi ion. REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 63—141   1. On what general principle are most of the telephone transmitters of today constructed?   2. Make sketch of the new Western Electric transmitter and describe its working.   3. Make sketch and describe the Kellogg transmitter.   4. What troubles were encountered in the earlier forms of granular carbon transmitters and how were they overcome?   5. What limits the current-carrying capacity of the transmitter? How may this capacity be increased?   6. State in what kind of transmitters a maximum degree of sensitivene is desirable.   7. Show the conventional symbols for transmitters.   8. Describe a telephone receiver.   9. Sketch a Western Electric receiver and point out its deficiencies. 10. Make a diagram of the Kellogg receiver. 11. Describe the direct-current receiver of the Automatic Electric Company. 12. Describe and sketch the Dean receiver. 13. Show the conventional symbols of a receiver. 14. Describe exactly how, in a cell composed of a tin and a silver plate with dilute sulphuric acid as electrolyte, the current inside and outside of the cell will flow. 15. Describe the phenomenon of polarization. 16. What is local action of a cell? How may it be prevented? 17. Into how many cla es may cells be divided? Which cla is most used in telephony? 18. Describe the LeClanché cell. 19. Sketch and describe an excellent form of dry cell. 20. Show the conventional symbols for batteries. 21. Sketch and describe the generator shunt switch and the generator cut-in switch. 22. How may a pulsating current be derived from a magneto generator? 23. Show conventional symbols for magneto generators. 24. Sketch and describe the Western Electric polarized bell. 25. Give conventional ringer symbols. 26. What is the purpose of the hook switch? 27. Make sketch and give description of Kellogg's long lever hook switch. 28. Describe and sketch the Western Electric short lever hook switch. 29. Point out the principal difference between the desk stand hook switches of the Western Electric Company and of the Kellogg Switchboard and Supply Company. 30. Give conventional symbols of hook switches. REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 143—225   1. Describe an electromagnet and its function in telephony.   2. Sketch an iron-clad electromagnet.   3. What is a differential electromagnet? Sketch and describe one type.   4. State the desirable characteristics of good enamel insulation for magnet wire.   5. If you have a coil of No. 23 double cotton B. and S. wire of 115 ohms resistance and you have to rewind it for 1,070 ohms resistance with double cotton wire, what number of wire would you take? Show calculation. **NOTE** No. 23 d. c wire has res. 1.772 ohms per cubic inch; for the core, 115 ohms. There are required in the coil 1,070 ohms, that is, 9.3 times as much. 1.772 x 9.3 = 16.47 ohms, which must be the resistance per cu. in. This resistance gives, according to Table IV, No. 29 wire.   6. What is an impedance coil? State how it differs from an electromagnet coil.   7. Describe the different kinds of impedance coils.   8. Give symbol of impedance coil.   9. What are the principal parts of an induction coil? 10. What is the function of an induction coil in telephony? 11. What is a repeating coil and how does it differ from an induction coil? 12. Give conventional symbols of induction coils and repeating coils. 13. Enumerate the different types of non-inductive resistance devices and give a short description of each. 14. Define condenser. 15. What is the meaning of the word dielectrics ? 16. State what you understand by the specific inductive capacity of a dielectric. 17. Upon what factors does the capacity of a condenser depend? 18. What is the usual capacity of condensers in telephone practice? 19. Give conventional condenser symbols. 20. By what two methods may the current be supplied to a telephone transmitter? 21. Make sketch of local-battery stations with metallic circuit. 22. Sketch common-battery circuit in series with two lines. 23. State the objections against the preceding arrangement. 24. Make sketch of the standard arrangement of the Western Electric Company in bridging the common battery with repeating coils. 25. Sketch the arrangement of bridging the battery with impedance coils and state the purpose of the coils. 26. Make diagram of a common-source current supply for many lines with repeating coils and point out the travel of the voice currents. 27. Name the different parts which comprise a telephone set. 28. What is a magneto telephone? 29. Make diagram of the circuit of a series magneto set with receiver on the hook and explain how the different currents are flowing. 30. Show diagram of the Stromberg-Carlson magneto desk telephone circuit and describe its working. 31. Give sketch of the Stromberg-Carlson common-battery wall set circuit. 32. Describe briefly the micro telephone set. 33. Make sketch of the Monarch common-battery wall set. REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 227—286   1. What is a party line?   2. What is usually understood by private lines?   3. What problem is there to overcome in connection with party lines?   4. State the two general cla es of party-line systems.   5. Point out the defects of the series system.   6. Make sketch of a metallic bridging line and show the circuit for the voice currents.   7. What is a signal code?   8. Give cla ification of selective party-line systems with short definitions.   9. Describe the principle of selection by polarity and make sketch illustrating this principle. 10. Make diagram of the circuit of a four-party station with relay. 11. Describe the proce of tuning in the harmonic system. 12. What is the difference between the under-tune and in-tune systems? 13. Sketch circuit of Kellogg's harmonic system. 14. Illustrate the principle of a broken-line system by a sketch. 15. In what particulars does the party-line system in rural districts differ from that within urban limits? 16. Describe and sketch Pool's lock-out system. 17. Make diagram of the K.B. lock-out system. 18. What is the object of the ratchet in this system? 19. Make diagram of simplified circuits of Roberts system. 20. Sketch and describe Roberts latching key and connections. 21. Sketch circuits of bridging station for non-selective party line. 22. How would you arrange the signal code for six stations on a non-selective party line? 23. What is the limit of number of stations on a non-selective party line under ordinary circumstances? 24. State the objections against the party polarity system as shown in Fig. 172. 25. What are the advantages of the harmonic party-line system? 26. To how many frequencies is the harmonic system usually limited? 27. What can you say about the commercial succe of the step-by-step method? 28. State the principles of a lock-out party line. 29. For what purpose is a condenser placed in the receiver circuit of each station in the K.B. lock-out system? 30. How are the selecting relays in Roberts line restored to their normal position after a conversation is finished? 31. What are the objections against the Roberts system? REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 287—315   1. What are electrical hazards?   2. When is the lightning hazard least?   3. What actions can electricity produce? Which involves the greater hazard to the value of property?   4. When is a piece of apparatus called "self-protecting"?   5. Why must a protector for telephone apparatus work more quickly for a large current than for a small one?   6. State the general problem which heating hazards present with relation to telephone apparatus.   7. What is the most nearly universal electrical hazard?   8. Sketch and describe the saw-tooth lightning arrester.   9. Make diagram of the carbon-block arrester and state its advantages. 10. Describe a vacuum arrester. 11. Explain the reason for placing an impedance in connection with the lightning arrester. 12. What is the purpose of the globule of low-melting alloy in the Western Electric Company's arrester? 13. Why are not fuses good lightning arresters? 14. What is the proper function of a fuse? 15. Make sketch of a mica slip fuse. 16. Define sneak currents . 17. Make a diagram of a sneak-current arrester and describe its principles and working. 18. Describe a heat coil. 19. Sketch a complete line protection. 20. Where is the proper position of the fuse? 21. Which wires are considered exposed and which unexposed? 22. Why is it not nece ary to install sneak-current arresters in central-battery subscribers' stations? 23. Sketch and describe the action of a combined sneak-current and air-gap arrester, as widely used by Bell companies. 24. Describe the self-soldering heat-coil arrester. 25. What is the purpose of ribbon fuses? 26. What is a drainage coil? REVIEW QUESTIONS ON THE SUBJECT OF TELEPHONY PAGES 317—386   1. What is a central office?   2. What are ( a ) subscriber's lines? ( b ) Trunk lines? ( c ) Toll lines?   3. For what purpose is the switchboard?   4. Give short descriptions of the different cla es of switchboards.   5. How are manual switchboards subdivided? Describe briefly the different types.   6. Define A and B boards.   7. What is a call circuit?   8. What kind of calls are handled on a toll switchboard?   9. Give drop symbol and describe its principles. 10. What is a jack? 11. Make a sketch of a plug inserted into a jack. 12. Give jack and plug symbols. 13. What are ringing and listening keys? 14. Show symbols for ringing and listening keys. 15. State the parts of which a cord equipment consists. 16. Show step by step the various operations of a telephone system wherein the lines center in a magneto switchboard. Make all the nece ary diagrams and give brief descriptions to show that you understand each operation. 17. On what principle does a drop with night-alarm contact operate? 18. What is the advantage of a ociating jacks and drops? 19. Describe the mechanical restoration as employed in the Miller drop and jack. 20. Describe the electrical restoration of drop shutters as manufactured by the Western Electric Company. 21. What complications arise in ringing of party lines and how are they overcome? 22. Give diagram of the complete circuit of a simple magneto switchboard. 23. Sketch night-alarm circuit with relay. 24. What is a convertible cord circuit? 25. State what disadvantages may be encountered under certain conditions with a bridging drop-cord circuit. 26. Are lamps in cord circuits to be advocated on magneto switchboards? 27. What is the function of the cabinet? 28. Give cro -section of upright switchboard as used in the magneto system. 29. What is the purpose of a sectional switchboard? 30. Give a short description of the e ential parts of a sectional switchboard. iNDEX ToC INDEX A Acoustic on transmitter   77 Acoustics   19 characteristics of sound   19 loudne   20 pitch   20 timbre   21 human ear   23 human voice   22 propagation of sound   19 Air-gap vs. fuse arresters   304 Amalgamated zincs   97 Arrester separators   296 Audible signals   39 magneto bell     40 telegraph sounder   39 telephone receiver   43 vibrating bell   40 Automatic Electric Company direct-current receiver   85 transmitter   72 Automatic shunt   124 B Bar electromagnet   148 Battery bell   115 Battery symbols   114 Blake single electrode   64 Brazed bell   130 Broken-back ringer   280 Broken-line method of selective signaling   261 , 275 C Capacity reactance   55 Carbon   31 adaptability   31 limitations   33 preparation of   75 superiority   32 Carbon air-gap arrester   300 Carbon-block arrester   296 Carrying capacity of transmitter   76 Central-office protectors   310 Characteristics of sound   19 loudne   20 pitch   20 timbre   21 Chloride of silver cell   113 Closed-circuit cells   106 Closed-circuit impedance coil   164 Common-battery telephone sets   217 Condensers   178 capacity   178 charge   178 conventional symbols   184 definition of   180 dielectric   180 dielectric materials   182 functions   184 means for a orting current   185 sizes   183 theory   179 Conductivity of conductors   49 Conductors, conductivity of   49 Conventional symbols   91 Cook air-gap arrester   298 arrester     311 arrester for magneto stations   313 Crowfoot cell   106 Current supply to transmitters   186 common battery   188 advantages   188 bell substation arrangement   192 bridging battery with impedance coils   194 bridging battery with repeating coil   191 current supply from distant point   204 current supply over limbs of line in parallel   200 Dean substation arrangement   197 double battery with impedance coil   195 Kellogg substation arrangement   198 North Electric Company system   199 series battery   189 series substation arrangement   190 Stromberg-Carlson system   198 supply many lines from common source   202 repeating coil   202 retardation coil   204 local battery   187 D Dean drop and jack   344 receiver   88 wall telephone hook   137 Desk stand hooks   138 Kellogg   140 Western Electric   138 Dielectric   180 Dielectric materials   182 dry paper   182 mica   182 Differential electromagnet   151 Direct-current receiver   84 Drainage coils   315 E Electric lamp signal   44 Electrical hazards   287 Electrical reproduction of speech   24 carbon   31 conversion from sound waves to vibration of diaphragm   21 conversion from vibration to voice currents   24 conversion from voice currents to vibration   25 cycle of conversion   26 detrimental effects of capacity   36 early conceptions   28 electrostatic telephone   30 induction coil   34 limitations of magneto transmitter   29 loose contact principle   27 magneto telephone   26 measurements of telephone currents   36 variation of electrical pre ure   30 variation of resistance   30 Electrical signals   39 audible   39 magneto-bell   40 telegraph sounder   39 telephone receiver   43 vibrating bell   40 visible   43 electric lamp signal   44 electromagnetic signal   43 Electrodes 75 arrangement of   63 carbon preparation   75 multiple   65 single   64 Electrolysis 315 Electromagnetic method of measuring telephone currents 37 Electromagnetic signal 43 Electromagnets and inductive coils 143 , 171 conventional symbols   171 differential electromagnet   151 direction of armature motion   151 direction of lines of force   146 electromagnets   143 low-resistance circuits   147 horseshoe form   147 iron-clad form   149 special horseshoe form   150 impedance coils   163 kind of iron   164 number of turns   103 types closed-circuit   164 open-circuit   164 toroidal   165 induction coil   165 current and voltage ratios   166 design   166 functions   166 use and advantage   167 magnet wire   153 enamel   156 silk and cotton insulation   155 space utilization   156 wire gauges   154 magnetic flux   144 magnetization curves   145 magnetizing force   143 mechanical details   152 permeability   144 reluctance   147 repeating coil   168 winding methods   158 winding calculations   162 winding data   159 winding terminals   159 Electrostatic capacity   50 unit of   52 Electrostatic telephone  30 Enamel   156 F Five-bar generator   123 Fuller cell   109 G Galvani   92 Generator armature   119 Generator cut-in switch   125 Generator shunt switch   124 Generator symbols   127 Granular carbon   66 Gravity cell   106 H Hand receivers 90 Harmonic method of selective signaling 248 advantages   258 circuits   257 in-tune system   253 limitations   258 principles   248 tuning   250 under-tune system   250 Head receivers   90 Heat coil   306 Holtzer-Cabot arrester   300 Hook switch   132 automatic operation   133 contact material   135 design   134 desk stand hooks   138 Kellogg   140 Western Electric   138 purpose   132 symbols   141 wall telephone hooks   135 Dean   137 Kellogg   135 Western Electric   136 Horseshoe electromagnet   148 Human ear   23 Human voice   22 I Impedance coils   163 kind of iron   164 number of turns   163 symbols of   165 types closed-circuit   164 open-circuit   164 toroidal   165 Inductance vs. capacity   56 Induction coil   34 , 165 current and voltage ratios   166 design   166 functions   166 use and advantage   167 Inductive neutrality   174 Inductive reactance   55 Insulation of conductors   56 Introduction to telephony   11 Iron-clad electromagnet   149 Iron wire ballast   177 K Kellogg air-gap arrester   299 desk stand hook   140 drop and jack   342 receiver   83 ringer   129 transmitter   70 wall telephone hook   135 L Lalande cell   112 Lamp filament   176 Le Clanché cell   99 Lenz law   54 Line signals   325 Lines of force, direction of   146 Loading coils   59 Lock-out party-line systems   263 broken-line method   275 operation   282 Poole system   264 step-by-step system   267 Loudne of sound   20 Low-reluctance circuits   147 horseshoe form   147 iron-clad form   149 M Magnetic flux   144 Magnetization curves   145 Magnetizing force   143 Magneto bell   40 , 115 Magneto operator   116 Magneto signaling apparatus   115 armature   119 automatic shunt   124 battery bell   115 generator symbols   127 magneto bell   115 magneto generator   116 method of signaling   115 polarized ringer   128 pulsating current   126 ringer symbols   131 theory   117 Magneto switchboard   324 automatic restoration   341 mechanical   341 Dean type   344 Kellogg type   342 Monarch type   345 Western Electric type   313 circuits of complete switchboard   356 code signaling   346 commercial types of drops and jacks   335 early drops   335 jack mounting   338 manual vs. automatic restoration   340 methods of a ociating   339 night alarm   338 tubular drops   336 component parts   325 jacks and plugs   325 keys   327 line and cord equipments   328 line signal   325 operators' equipment   328 cord-circuit considerations   364 double clearing-out type   368 lamp-signal type   371 non-ring through type   366 series drop type   366 simple bridging drop type   364 definitions   324 electrical restoration   347 grounded and metallic-circuit lines   360 mode of operation   324 night-alarm circuits   358 operation in detail   328 clearing out   335 e entials of operation   335 normal condition of line   329 operator answering   330 operator calling   332 subscriber calling   330 subscribers conversing   333 operator's telephone equipment   354 cut-in jack   355 ringing and listening keys   351 horizontal spring type   351 party-line ringing keys   353 self-indicating keys   354 vertical spring type   352 switchboard a embly   373 functions of cabinet   373 sectional switchboards   384 upright type of switchboard   376 wall type switchboard   373 switchboard cords   349 concentric conductors   350 parallel tinsel conductors   351 steel spiral conductors   350 switchboard plugs   348 Magneto telephone   26 Magneto telephone sets   209 Mica card resistance   174 Mica slip fuse   302 Micro telephone set   223 Monarch drop and jack   345 Monarch receiver   86 Monarch transmitter   73 Multiple electrode   65 Mutual induction   54 N Non-inductive resistance devices   172 inductive neutrality   174 provisions against heating   174 temperature coefficient   172 types differentially-wound unit   175 iron wire ballast   177 lamp filament   176 mica card unit   174 Non-selective party-line systems   227 bridging   230 limitations   235 series   229 signal code   233 O Open-circuit cells   99 Open-circuit impedance coil   164 Operator's receiver   90 P Packing of transmitters   75 Permeability   144 Pitch   20 Doppler's principle   20 vibration of diaphragms   21 Polarity method of selective signaling   239 Polarization of cells   96 Polarized ringer   41 , 128 brazed bell   130 Kellogg   129 Western Electric   128 Poole lock-out system   264 Primary cells   92 conventional symbol   114 series and multiple connections   98 simple voltaic   92 types of closed-circuit   106 Fuller   109 gravity   106 Lalande   112 prevention of creeping   106 setting up   108 open-circuit   99 Le Clanché   99 standard   113 chloride of silver   113 Propagation of sound   19 Protective means   294 against high potentials   294 air-gap arrester   295 advantages of carbon   296 commercial types   297 continuous arcs   300 discharge acro gaps   295 dust between carbons   296 introduction of impedance   299 metallic electrodes   300 vacuum arresters   299 against sneak currents   304 heat coil   306 sneak-current arresters   305 against strong currents   301 fuses   301 enclosed   302 mica   301 proper functions   301 central-office protectors   310 self-soldering heat coils   310 sneak-current and air-gap arrester   310 city exchange requirements   314 complete line protection   306 electrolysis   315 subscribers' station protectors   312 ribbon fuses   313 Pulsating-current commutator   127 R Receivers   80 Dean   88 direct-current   84 early   80 Kellogg   83 modern   81 Monarch   86 operator's   90 single-pole   81 symbols   91 Western Electric   82 Reluctance   147 Repeating coil   168 Ribbon fuses   313 Ringer symbols   131 Ringing and listening key   327 Robert's latching relay   276 Robert's self-cleansing arrester   297 Rolled condenser   182 S Saw-tooth arrester   296 Selective party-line systems   238 broken-line method   261 cla ification broken-line systems   239 harmonic systems   238 polarity systems   238 step-by-step systems   239 harmonic method   248 polarity method   239 step-by-step method   259 Self-induction   54 Signal code   235 Signaling, method of   115 Silk and cotton insulation   155 Single electrode   64 Single-pole receiver   81 Sneak-current arresters   305 Solid-back transmitter   66 Sound characteristics of   19 loudne   20 pitch   20 timbre   21 Standard cell   113 Step-by-step lock-out system   67 Step-by-step method of selective signaling   259 Subscribers' station protectors   312 Switchboard cords   349 Switchboard plugs   348 Switchboard transmitter   79 Symbols battery   114 condenser   184 generator   127 hook switch   141 impedance coil   165 induction coil   171 receiver   91 repeating coil   171 ringer   131 ringing and listening key   327 transmitter   79 T Table condenser data   184 copper wire   151 German silver wire — 18 per cent   174 German silver wire — 30 per cent   175 metals, behavior of, in different electrolysis   95 signal code   235 specific inductive capacities   181 temperature coefficients   172 transmi ion distances, limiting   57 winding data for insulating wires   160 Tandem differential electromagnet   152 Telegraph sounder   30 Telephone currents, measurements of   36 electromagnetic method   37 thermal method   37 Telephone exchange, features of   317 districts   318 subscribers' lines   318 switchboards   319 toll lines   318 trunk lines   318 Telephone lines   47 conductivity of conductors   49 electrostatic capacity   50 inductance of circuit   54 inductance vs. capacity   56 insulation of conductors   56 transmi ion   60 Telephone sets   207 cla ification of   208 common-battery telephone   208 magneto telephone   208 wall and desk telephones   208 common-battery   217 desk   220 hotel   219 wall   219 magneto   209 circuits of bridging   215 series   212 desk   211 wall   209 Temperature coefficients   172 Thermal method of measuring telephone currents  37 Timbre   21 Toroidal impedance coil   165 Toroidal repeating coil   170 Transmi ion, ways of improving   60 Transmitters   63 acoustic on   77 Automatic Electric Company   72 carrying capacity   76 conventional diagram   79 electrode   75 arrangement of   63 multiple   65 single   64 granular carbon   66 Kellogg   70 materials   63 Monarch   73 packing   75 sensitivene   77 switchboard   79 symbols   79 variable resistance   63 Western Electric solid-back   66 U Under-tuned ringer   252 V Vacuum arrester   299 Variable resistance   63 Vibrating bell   40 Visible signals   43 electric lamp   44 electromagnetic   43 Volta   92 Voltaic cell   92 amalgamated zincs   97 difference of potential   94 local action   96 polarization   96 theory   93 W Wall telephone hooks   135 Dean   137 Kellogg   135 Western Electric   136 Western Electric air-gap arrester   298 desk stand hook   138 drop and jack   343 receiver   82 ringer   128 solid-back transmitter   66 station arrester   313 wall telephone hook   136 White transmitter   66 Wire gauges   154 Updated editions will replace the previous one—the old editions will be renamed. 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