SELF-RECORDING RADIOGRAPH 



199 



moving paper by means of electric sparks. These different 

 operations are carried out in proper sequence by the auto- 

 matic devices described below. 



Kj completes the battery circuit for about 10 seconds, 

 by which time the record is completed. The successive 





Fig. 115. The Self-Recording Radiograph. 



The selenium cell, s, is periodically exposed to light by the electro- 

 magnetic shutter, t. The selenium cell forms one arm of the 

 Wheatstone Bridge, b. The three keys, Kj, K,, K3, are 

 periodically closed and opened by clockwork, g, the 

 recording galvanometer with index, i, carrying double- 

 pointed sparking platinum wire, which moves between the 

 metal strip c and the plate m. r, sparking coil with its 

 electrode connected with c and m. The battery is not 

 shown in the figure {see text). 



records of variation of light are taken at intervals of 

 15 minutes ; the periodic closures of the circuit are thus for 

 10 seconds at intervals of 15 minutes. In practice, this 

 short passage of the current is found to cause no polarisation. 

 The second key, K2, actuates an electro-magnetic device 

 by which the trap-door T is opened for the definite period 

 of I second; the selenium cell, S, inside the dark box is 

 thus exposed to light for this length of time. The trap-door 



