THE MAGNETO TELEPHONE 125 



The next step, therefore, in the development of the telephone circuit consisted in 

 th.e introduction into it of a voltaic battery and a carbon microphone or transmitter 

 as shown in .Fig. 75. 



The back electrode B is a small metal case held to the support A by a lock screw. 

 It carries in it the metal plate : C, which bears a carbon face. Over this is a similar 

 plate D and between the two is placed a quantity of carbon granules. The plate D 

 carries the screw-threaded bars E and G. Over these fits the mica washer F, the nut IT, 

 and the screw-cap I. The bar G projects through the vibratory diaphragm J, which is 

 held in place by the nuts S and S. 



The transmitter is perhaps the .most delicate portion of the modern telephone set. 

 It is readily burned out by atmospheric electricity, and the carbon granules sometimes 

 pack together, especially if they become damp, thus interfering with transmission. Xo 

 one but an expert instrument man should ever undertake to repair an injured' trans- 

 mitter. 



3 BATTERY 



For telephone service what is known as an open-circuit cell is preferred as a source 

 of electric current for talking purposes. As a rule a battery of two or three such cells 

 is used. It might seem that transmission could be improved by the use of a very strong 

 current, but the difficulty arises that such currents heat the carbon contact in the 

 transmitter, and, in time, ruin it. For this reason low-resistance, low-voltage batteries 

 are preferred. 



Open-circuit cells are those which have the power of recuperating through 

 depolarization whenever the circuit is left open. They are not suitable for prolonged 

 continuous use, as are closed-circuit cells, which work best when in constant use. As 

 will be later explained, the talking circuit of a telephone is always open except when 

 the telephone is in use or the receiver off the hook and, therefore, open-circuit cells are 

 the best for this service. The greater convenience of dry cells has led to their general 

 adoption for telephone purposes. These are in all respects like ordinary cells except 

 that the liquid solutions are mixed with some absorbent material and so rendered 

 practically solid. There is thus little danger of liquids spilling or leaking out and 

 causing damage. At the same time this danger is not entirely absent and batteries 

 should never be carried in telephone sets, especially when lying on their sides. In this 

 position, if there is any defect in the shell of the battery, some of the contents may leak 

 out and corrode the wiring and other metal parts with which they come in contact. 

 Dry cells, of course, become exhausted in time and must be renewed just as do other 

 forms of primary cells. 



4 INDUCTION COIL 



The next important step in the development of the telephone after the invention 

 of the transmitter was the introduction of an induction coil into the circuit 'by Edison. 

 This, in fact, marked the beginning of practical telephone development, since, prior 

 to the use of an induction coil, the range of transmission was extremely limited. Even 

 after the transmitter was employed, it was found that the changes produced in the total 

 line resistance by the varying pressure on the carbons were so small in comparison t< 

 the total line resistance as to produce variations in the current that were scarcely per- 

 ceptible in a distant receiver. The problem, therefore, was to arrange some means by 

 which the voltage of the current going out over the line could be increased, other than 

 by increasing the voltage of the battery, which we have already seen had 

 limitations. 



It had further been noted that the telephone receiver was more sensitive to minut 

 alternating currents than to the varying- direct currents such as had up to this 

 been employed, but no means by which an alternating current might be employee 

 been devised. 



