THE AUTOMATIC KEYS 



101 



exposed to light, and the resulting deflection gives a measure 

 of the intensity of the light. 



The Automatic Keys 



After previous adjustment of the balance in the dark, 

 the electric circuit is completed by the closure of key K lf after 



Fig. 29. 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, k v k 2 , k 3 , are 

 periodically closed and opened by clockwork, g, the record- 

 ing galvanometer with index, 1, carrying a double-pointed 

 platinum wire at its end, which moves between the metal 

 strip, c, and the plate, m. p, paper for record. R, sparking 

 coil with its electrodes connected with c and M. The battery 

 is not shown in the figure (see text). 



which the selenium cell is exposed to light by an automatic 

 electro-magnetic shutter. The deflection of the galvano- 

 meter is recorded on a moving sheet of paper P by means of 

 electric sparks. These different operations are carried out 

 in proper sequence by the automatic devices described below. 

 Key k x completes the battery circuit for about 10 seconds, 



