MICROPHONE TRANSMITTER 123 



It will at once be seen that in this procedure we 

 are dealing with currents of very low magnitudes, 

 and Belin found that considerable modification of 

 his apparatus was necessary. In the first place the 

 relief in the photographs as used for his original 

 machines was much too great, and whereas very 

 thickly -coated tissue difficult to print and slow to 

 dry had been at first used, he now uses very thin 

 tissue, so that the different thicknesses of gelatine 

 which correspond to different tones are very minute. 

 If we take the curve showing the relation between 

 the distance of the microphone diaphragm from the 

 magnet poles and the current strength at the 

 receiver, it is found that only a short portion of 

 such curve is suitable for the system, and much 

 variation (and hence much thickness of film) is 

 impossible in the position of his diaphragm. The 

 microphone used is a large form of carbon instru- 

 ment, in which the pressure of the diaphragm on 

 three carbon balls varies their resistance, and 

 therefore the strength of the current flowing 

 through the circuit. With a large instrument of 

 this type he can allow of much greater variation in 

 the position of the diaphragm than is usual in the 

 diaphragm of an ordinary telephone ; the latter, 

 measured at the centre, being something like io~ 6 

 cm. for a just audible sound. 



By making use of a bridge arrangement, the 

 balance of which is upset by the alteration in resist- 



