Circuits of Magneto-Telephone Sets

Magneto telephones, whether of the wall or desk type, may be divided into two general cla es, series and bridging, according to whether the magnet of the bell is included in series or bridge relation with the telephone line when the hook is down. Series. In the so-called series telephone line, where several telephones are placed in series in a single line circuit, the employment of the series type of telephone results in all of the telephone bells being in series in the line circuit. This means that the voice currents originating in the telephones that are in use at a given time must pa in series through the magnets of the bells of the stations that are not in use. In order that these magnets, through which the voice currents must pa , may interfere to as small a degree as po ible with the voice currents, it is common to employ low-resistance magnets in series telephones, these magnets being wound with comparatively few turns and on rather short cores so that the impedance will be as small as po ible. Likewise, since the generators are required to ring all of the bells in series, they need not have a large current output, but must have sufficient voltage to ring through all of the bells in series and through the resistance of the line. For this reason the generators are usually of the three-bar type and sometimes have only two bars. In Fig. 146 are shown, in simplified form, the circuits of an ordinary series telephone. The receiver in this is shown as being removed from the hook and thus the talking apparatus is brought into play. The line wires 1 and 2 connect respectively to the binding posts 3 and 4 which form the terminals of the instrument. When the hook is up, the circuit between the binding posts 3 and 4 includes the receiver and the secondary winding of the induction coil, together with one of the upper contacts 5 of the switch hook and the hook lever itself. This completes the circuit for receiving speech. The hook switch is provided with another upper contact 6 , between which and the contact 5 is connected the local circuit containing the transmitter, the battery, and the primary of the induction coil in series. The primary and the secondary windings are connected together at one end and connected with the switch contact 5 , as shown. It is thus seen that when the hook is up the circuit through the receiver is automatically closed and also the local circuit containing the primary, the battery, and the transmitter. Thus, all the conditions for transmitting and receiving speech are fulfilled. IMG:384091999325875926_fig146_t.gif.png:Illustration_ Fig. 146. Circuit of Series Magneto Set Fig. 146. Circuit of Series Magneto Set View full size illustration. When the hook is down, however, the receiving and transmitting circuits are broken, but another circuit is completed by the engagement of the hook-switch lever with the lower hook contact 7 . Between this contact and one side of the line is connected the polarized ringer and the generator. With the hook down, therefore, the circuit may be traced from the line wire 1 to binding post 3 , thence through the generator shunt to the call bell, and thence through the lower switching contact 7 to the binding post 4 and line wire 2 . The generator shunt, as already described in Chapter VIII, normally keeps the generator shunted out of circuit. When, however, the generator is operated the shunt is broken, which allows the armature of the generator to come into the circuit in series with the winding of the polarized bell. The normal shunting of the generator armature from the circuit of the line is advantageous in several ways. In the first place, the impedance of the generator winding is normally cut out of the circuit so that in the case of a line with several stations the talking or voice currents do not have to flow through the generator armatures at the stations which are not in use. Again, the normal shunting of the generator tends to save the generator armature from injury by lightning. IMG:384091999325875926_fig147_t.gif.png:Illustration_ Fig. 147. Circuit of Series Magneto Set Fig. 147. Circuit of Series Magneto Set View full size illustration. The more complete circuits of a series magneto telephone are shown in Fig. 147. In this the line binding posts are shown as 1 and 2 . At the bottom of the telephone cabinet are four other binding posts marked 3 , 4 , 5 , and 6 . Of these 3 and 4 serve for the receiver terminals and 5 and 6 for the transmitter and battery terminals. The circuits of this diagram will be found to be e entially the same as those of Fig. 146, except that they are shown in greater detail. This particular type of circuit is one commonly employed where the generator, ringer, hook switch, and induction coil are all mounted in a so-called magneto bell box at the top of the instrument, and where the transmitter is mounted on an arm just below this box, and the battery in a separate compartment below the transmitter. The only wiring that has to be done between the bell box and the other parts of the instrument in a embling the complete telephone is to connect the receiver to the binding posts 3 and 4 and to connect the battery and transmitter circuit to the binding posts 5 and 6 . Bridging. In other cases, where several telephones are placed on a single-line circuit, the bells are arranged in multiple acro the line. For this reason their magnets are wound with a very great number of turns and consequently to a high resistance. In order to further increase the impedance, the cores are made long and heavy. Since the generators on these lines must be capable of giving out a sufficient volume of current to divide up between all of the bells in multiple, it follows that these generators must have a large current output, and at the same time a sufficient voltage to ring the bells at the farthest end of the line. Such instruments are commonly called bridging instruments, on account of the method of connecting their bells acro the circuit of the line. IMG:384091999325875926_fig148_t.gif.png:Illustration_ Fig. 148. Circuit of Bridging Magneto Set Fig. 148. Circuit of Bridging Magneto Set View full size illustration. The fundamental characteristic of the bridging telephone is that it contains three po ible bridge paths acro the line wires. The first of these bridge paths is through the talking apparatus, the second through the generator, and the third through the ringer. This is shown in simplified form in Fig. 148. The talking apparatus is a ociated with the two upper contacts of the hook switch in the usual manner and needs no further description. The generator is the second separate bridge path, normally open, but adapted to be closed when the generator is operated, this automatic closure being performed by the movement of the crank shaft. The third bridge contains the polarized bell, and this, as a rule, is permanently closed. Sometimes, however, the arrangement is such that the bell path is normally closed through the switch which is operated by the generator crank shaft, and this path is automatically broken when the generator is operated, at which time, also, the generator path is automatically closed. This arrangement brings about the result that the generator never can ring its own bell, because its switch always operates to cut out the bell at its own station just before the generator itself is cut into the circuit. In Fig. 149 is shown the complete circuit of a bridging telephone. The circuit given in this figure is for a local-battery wall set similar in type to that shown in Figs. 142 and 143. A simplified diagrammatic arrangement is shown in the lower left-hand corner of this figure, and from a consideration of this it will be seen that the bell circuit acro the line is normally completed through the two right-hand normally closed contacts of the switch on the generator. When, however, the generator is operated these two contacts are made to disengage each other while the long spring of the generator switch engages the left-hand spring and thus brings the generator itself into the circuit. IMG:384091999325875926_fig149_t.gif.png:Illustration_ Fig. 149. Circuit of Bridging Magneto Set Fig. 149. Circuit of Bridging Magneto Set View full size illustration. Of the three binding posts, 1 , 2 , and 3 , at the top of Fig. 149, 1 and 2 are for connecting with the line wires, while 8 is for a ground connection, acting in conjunction with the lightning arrester mounted at the top of the telephone and indicated at 4 in Fig. 149. This has no function in talking or ringing, and will be referred to more fully in Chapter XIX. Suffice it to say at this point that these arresters usually consist of two conducting bodies, one connected permanently to each of the line binding posts, and a third conducting body connected to the ground binding post. These three conducting bodies are in close proximity but carefully insulated from each other; the idea being that when the line wires are struck by lightning or subjected otherwise to a dangerous potential, the charge on the line will jump acro the space between the conducting bodies and pa harmle ly to ground. **NOTE** The student should practice making simplified diagrams from actual wiring diagrams. The difference between the two is that one is laid out for ease in understanding it, while the other is laid out to show the actual course of the wires as installed. If the large detailed circuit of Fig. 149 be compared with the small theoretical circuit in the same figure, the various conducting paths will be found to be the same. Such a simplified circuit does more to enable one to grasp the fundamental scheme of a complex circuit than much description, since it shows at a glance the general arrangement. The more detailed circuits are, however, nece ary to show the actual paths followed by the wiring. The circuits of desk stands do not differ from those of wall sets in any material degree, except as may be nece itated by the fact that the various parts of the telephone set are not all mounted in the same cabinet or on the same standard. To provide for the nece ary relative movement between the desk stand and the other portions of the set, flexible conductors are run from the desk stand itself to the stationary portions of the equipment, such as the battery and the parts contained in the generator and bell box. IMG:384091999325875926_fig150_t.gif.png:Illustration_ Fig. 150. Circuit of Bridging Magneto Desk Set Fig. 150. Circuit of Bridging Magneto Desk Set View full size illustration. In Fig. 150 is shown the circuit of the Stromberg-Carlson magneto desk-telephone set, illustrated in Fig. 145. This diagram needs no explanation in view of what has already been said. The conductors, leading from the desk-stand group of apparatus to the bell-box group of apparatus, are grouped together in a flexible cord, as shown in Fig. 145, and are connected respectively to the various binding posts or contact points within the desk stand at one end and at the base of the bell box at the other end. These flexible conductors are insulated individually and covered by a common braided covering. They usually are individualized by having a colored thread woven into their insulating braid, so that it is an easy matter to identify the two ends of the same conductor at either end of the flexible cord or cable. IMG:384091999325875926_fig151_t.gif.png:Illustration_ Fig. 151. Common-Battery Wall Set Fig. 151. Common-Battery Wall Set View full size illustration. IMG:384091999325875926_fig152_t.gif.png:Illustration_ Fig. 152. Common-Battery Wall Set Fig. 152. Common-Battery Wall Set View full size illustration.