840 Mr. T. Thome Baher [April 22, 



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

 all systems of photo-telegraphy. The motors which drive each drum 

 are run at about 3000 revolutions per minute, and geared down 

 YQYj considerably, so that the drums themselves revolve, perhaps, at 

 30 revolutions per minute ; the motors are run from secondary 

 batteries of ample capacity to ensure 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, which 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 alterna- 

 tions 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 trans- 

 mitting drum. It consequently completes its revolution before the 

 transmitter. 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 

 polarised relay which actuates an electromagnet, and this magnet 

 removes the check. 



Thus, however much one drum gets out of step with the other, 

 the fault is limited to each revolution, and both di'ums must always 

 start off in unison for each new revolution. I have found that where 

 each operator endeavours to keep his motor running uniformly by 

 regulating the resistance according to the fluctuations recorded by 

 the frequency meter, the personal element makes itself visible in the 

 results ; straight hues appear wavy, and the synchronism is not at all 

 good. I therefore tried very carefully calibrating the motors by 

 timing first, and then arranged that, once started, the motors should 

 not be touched ; the gain in speed of each is approximately the same 

 if both motors are run from secondary batteries of the same ampere- 

 hour capacity, and in this way we have obtained the most perfect 

 results as regards synchronisation. 



The great advantage of this process is that the whole operation 

 is in full view, whereas with systems in which the received picture 

 is obtained on a photographic film one has to develop such film 

 before it is possible to discover whether anything is wrong. "With 

 the receiver described, the operator keeps his hand on the sliding 

 contact of the resistances, and merely adjusts their position during 

 the first two or three seconds according to the condition of the 

 eleotrolytic marks, i.e., whether crisp and concise or not. The 



