TELEGRAPHY OF PHOTOGRAPHS BAKER. 261 



iuiportant function or interesting event can take the machine with 

 him, prepare his pictures, and telegraph tliem to his head office, and 

 when the event is over he simply returns with the apparatus. For 

 criminal investigations, the portable instrument Avill, I feel sure, be- 

 come of considerable value also. Through the continued courtesy 

 shown by the Postmaster General and Maj. O'Meara, the engineer in 

 chief, we have been given every facility for developing the work, 

 and I believe that the uses of the portable instrument will before long 

 have been amply demonstrated. 



If a picture revolving beneath a tracer has to redraw itself, as it 

 were, on a piece of paper perhaps hundreds of miles away, it is 

 obvious that each mark redrawn must occupy a precisely similar spot 

 on the new j^aper as it does in the original picture. As cylinders or 

 drums are used in picture telegraphy, this means that they must re- 

 volve in perfect unison. If one drum w'ere to gain on the other we 

 should have, in the case of a portrait, a nose being recorded where 

 the eye ought to be, or something equally disastrous; in fact, if the 

 two machines get the least bit out of step, the received picture is com- 

 pletely ruined. The method of synchronising used by Prof. Korn 

 has proved very satisfactory, and has been adopted in practically all 

 systems of phototelegraphy. The motors which drive each drum are 

 run at about 3,000 revolutions per minute, and geared down very con- 

 siderably, so that the drums themselves revolve, perhaps, at 30 revo- 

 lutions per minute; the motors are run from secondary batteries of 

 ample capacity to insure smooth working, and should be run for a 

 sufficient time before beginning a transmission to allow of their 

 warming up. 



The speed of each motor is controlled by a regulating resistance in 

 series with the field magnets, and the speed is ascertained by means 

 of a frequency meter, wdiich indicates the number of revolutions per 

 second. The dial of this meter is shown on the screen. A set of tuned 

 steel tongues are fixed in front of a magnet, which is supplied with 

 alternating current obtained from slip rings on the motor, and each 

 tongue has a different period of vibration. When the alternations 

 in magnetism correspond with the period of vibration of any one 

 spring, that spring vibrates, and thus serves as an indication of the 

 speed of the motor. 



The receiving drum is revolved a little quicker than the transmit- 

 ting drum. It consequently completes its revolution before the trans- 

 mitter. It is then stopped by a steel check, and is obliged to wait 

 until the other drum has caught it up. When the transmitting drum 

 has completed its turn, a fleeting contact comes into play, a reverse 

 current is sent to the receiving instrument; this is led into a polar- 

 ized relay, which actuates an electromagnet, and this magnet re- 

 moves the check. 



