42 PHOTO-TELEGRAPHY 



seconds, or in reality at about T 9 o 9 oths of this speed ; as 

 the receiving drum must revolve quicker, in order 

 to be stopped and then restarted by the synchronism 

 signal of the transmitter after each complete revolu- 

 tion, it is run at about xui tns f the speed. There is 

 thus a difference of 2 per cent, in the rates of the 

 two, which means an unavoidable elongation of the 

 received image. 



The regulation of the motors is carried out by 

 means of regulating resistances and frequency 

 metres. The motor acts to a small extent as a 

 dynamo, as two sections of the armature are con- 

 nected to slip rings with brushes fitted, so that an 

 alternating current is derived from them. This 

 alternating current is supplied to an electro- 

 magnet, whose polarity at one pole -piece therefore 

 changes twice in each revolution of the armature. 

 A row of tuned steel magnetised springs is fixed 

 in front of the pole -piece, fixed at the base and free 

 to vibrate vertically. These springs are thus 

 attracted and repulsed at each revolution of the 

 motor, and if a certain spring among them be cut to 

 such a length that its period is equal to the period of 

 the alternating current, it will vibrate very freely, 

 while other springs not in tune will not vibrate at all . 



Now several such springs, so cut that their 

 periods range from 97 per second to 103 per 

 second, are fixed at their lower ends in front of the 

 pole -piece pf the electro -magnet. By means of the 



