4o8 POPULAR SCIENCE MONTHLY 



It does seem to me that the physical principle used by Edison in his carbon 

 telephone, and by Hughes in the microphone, is the same, and that it is the same 

 as that used by M. Clerac in the variable resistance carbon tubes which he had 

 given to Mr. Hughes and others for important practical applications as early 

 as 1866, and that it depends entirely on the fact, long ago pointed out by 

 du Moncel, that increase of pressure between two conductors in contact pro- 

 duces diminution of electric resistance between them. 



Bell's hand transmitter was not only a telephone complete in itself, 

 but was a self-contained generator of the alternating-current type. 

 It was operated by the voice creating sound waves that in turn gen- 

 erated electric waves through the movements of the diaphragm. These 

 electrical waves were similar in form to the sound waves and were 

 transmitted to the receiver and there changed back to sound waves. 

 When in operation the flow of the current, and every variation in its 

 strength, was dependent on the varying motions of a diaphragm 

 moving in a magnetic field; that is, on the speed of an armature of 

 a miniature dynamo driven by the spoken word. In other words, in 

 the hand telephone respondent vibratory motion of a soft iron induc- 

 tion armature in a magnetic field was the essential element in the 

 successful transmission of speech. 



In the Blake and other forms of variable resistance transmitters, 

 whether single or multi-contact, there is no electro-magnet and no 

 armature. A battery, usually of the sal-ammoniac type, supplies a 

 constant current, the flow of which is regulated by increasing or 

 decreasing the pressure of the diaphragm against the carbon button, 

 the changes in pressure being governed by the impact of the chan- 

 ging sound waves on the diaphragm. Thus a carbon transmitter is 

 not so sensitive, nor does it possess that delicate responsiveness so 

 noticeable in a magneto transmitter. It matters little what may be 

 the nature or character of the diaphragm in a variable resistance trans- 

 mitter, so long as it is sensitive enough to reciprocally respond to the 

 sound waves produced by the vocal cords. But only a soft iron in- 

 ductive diaphragm will serve in the magneto type of transmitter. 



In the White or solid-back transmitter, now so familiar a part of 

 Bell equipment, the single-contact feature of the Blake transmitter 

 is succeeded by a multi-contact arrangement composed of two carbon 

 electrodes made of the hardest of pure carbon separated by carbon 

 granules. The selected granules insure a multitude of contacts, and 

 talking qualities that are unexcelled. 



In all these variable-contact transmitters the current is always 

 knocking at the carbon gateway and seeping through. When the tele- 

 phone is not in use, the carbon offers just sufficient resistance to pre- 

 vent the current from forcing the gate wide open. When a person is 

 talking, the vibrations of the diaphragm decrease the resistance of the 

 carbon and enable the current to flow through the partially or wholly 

 opened gateway. 



