122 PHOTO-TELEGRAPHY 



models he has arranged that the cylinders may be 

 driven at one of three different speeds, so that the 

 transmission can be effected rapidly or slowly as 

 desired. This is a very useful feature for experi- 

 mental instruments. 



The method of employing a rheostat and travel- 

 ling wheel has now been more or less abandoned, 

 and he has adapted to the instruments a special 

 form of microphone, which has given some remark- 

 ably good results over artificial lines, I.e., under 

 laboratory conditions. 



It is of course well known that if a diaphragm of 

 iron be fixed near the poles of a magnet, the mag- 

 netic lines of force pass through the diaphragm. 

 If the diaphragm be brought nearer to or further 

 from the poles, a change in the magnetic field takes 

 place. If small coils be wound round the pole- 

 pieces and connected in series with each other and 

 with a battery and telephone, then any shift in the 

 position of the diaphragm will be noticed by a 

 sound in the telephone. Belin conceived the idea 

 of making the stylus press against the diaphragm 

 of the microphone, so that the pressure on it would 

 vary in accordance with the relief in the photo- 

 graphic image ; this would vary the magnetic field 

 and so change the power of a current of electricity 

 passed through the microphone in series with a 

 battery and the two wires of the Blondel 

 oscillograph. 



