Possible Ways of Improving Transmission

Practical ways of improving telephone transmi ion are of two kinds: to improve the lines and to improve the apparatus. The foregoing shows what are the qualities of lines and the ways they require to be treated. Apparatus treatment, in the present state of the art, is addre ed largely to the reduction of lo es. Theoretical considerations seem to show, however, that great advance in apparatus effectivene still is po ible. More powerful transmitters—and more faithful ones—more sensitive and accurate receivers, and more efficient translating devices surely are po ible. Discovery may need to intervene, to enable invention to restimulate. In both telegraphy and telephony, the longer the line the weaker the current which is received at the distant end. In both telegraphy and telephony, there is a length of line with a given kind and size of wire and method of construction over which it is just po ible to send intelligible speech or intelligible signals. A repeater, in telegraphy, is a device in the form of a relay which is adapted to receive these highly attenuated signal impulses and to re-transmit them with fresh power over a new length of line. An arrangement of two such relays makes it po ible to telegraph both ways over a pair of lines united by such a repeater. It is practically po ible to join up several such links of lines to repeating devices and, if need be, even submarine cables can be joined to land lines within practical limits. If it were nece ary, it probably would be po ible to telegraph around the world in this way. If it were po ible to imitate the telegraph repeater in telephony, attenuated voice currents might be caused to actuate it so as to send on those voice currents with renewed power over a length of line, section by section. Such a device has been sought for many years, and it once was quoted in the public pre that a reward of one million dollars had been offered by Charles J. Glidden for a succe ful device of that kind. The records of the patent offices of the world show what effort has been made in that direction and many more devices have been invented than have been patented in all the countries together. Like some other problems in telephony, this one seems simpler at first sight than it proves to be after more exhaustive study. It is po ible for any amateur to produce at once a repeating device which will relay telephone circuits in one direction. It is required, however, that in practice the voice currents be relayed in both directions, and further, that the relay actually augment the energy which pa es through it; that is, that it will send on a more powerful current than it receives. Most of the devices so far invented fail in one or the other of these particulars. Several ways have been shown of a embling repeating devices which will talk both ways, but not many a embling repeating devices have been shown that will talk both ways and augment in both directions. IMG:384091999325875926_fig036_t.gif.png:Illustration_ Fig. 36. Shreeve Repeater and Circuit Fig. 36. Shreeve Repeater and Circuit View full size illustration. Practical repeaters have been produced, however, and at least one type is in daily succe ful use. It is not conclusively shown even of it that it augments in the same degree all of the voice waves which reach it, or even that it augments some of them at all. Its action, however, is distinctly an improvement in commercial practice. It is the invention of Mr. Herbert E. Shreeve and is shown in Fig. 39. Primarily it consists of a telephone receiver, of a particular type devised by Gundlach, a ociated with a granular carbon transmitter button. It is further a ociated with an arrangement of induction coils or repeating coils, the object of these being to accomplish the two-way action, that is, of speaking in both directions and of preventing reactive interference between the receiving and transmitting elements. The battery 1 energizes the field of the receiving element; the received line current varies that field; the resulting motion varies the resistance of the carbon button and transforms current from battery 2 into a new alternating line current. By reactive interference is meant action whereby the transmitter element, in emitting a wave, affects its own controlling receiver element, thus setting up an action similar to that which occurs when the receiver of a telephone is held close to its transmitter and humming or singing ensues. No repeater is succe ful unle it is free from this reactive interference. IMG:384091999325875926_fig037_t.gif.png:Illustration_ Fig. 37. Mercury-Arc Telephone Relay Fig. 37. Mercury-Arc Telephone Relay View full size illustration. Enough has been accomplished by practical tests of the Shreeve device and others like it to show that the search for a method of relaying telephone voice currents is not looking for a pot of gold at the end of the rainbow. The most remarkable truth established by the succe of repeaters of the Shreeve type is that a device embodying so large inertia of moving parts can succeed at all. If this mean anything, it is that a device in which inertia is absolutely eliminated might do very much better. Many of the methods already proposed by inventors attack the problem in this way and one of the most recent and most promising ways is that of Mr. J.B. Taylor, the circuit of whose telephone-relay patent is shown in Fig. 37. In it, 1 is an electromagnet energized by voice currents; its varying field varies an arc between the electrodes 2-2 and 3 in a vacuum tube. These fluctuations are transformed into line currents by the coil 4 . ToC cHAPTER V