102 CHAP. XIII. THE AUTOMATIC RADIOGRAPH 



by which time the record is completed. The records regis- 

 tering the variations 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, k 2 , actuates an electro-magnetic device 

 by which the trap-door t is opened for the definite period of 

 1 second ; the selenium cell S inside the dark box is thus 

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

 is seen in the diagram in the roof of the dark box. In 

 reality it is at the upper end of a vertical tube the inside 

 of which is coated with lamp-black to prevent side-reflection. 

 The light that falls on the selenium cell is from a definite 

 area of the sky. The intensity of the light from the sky at 

 different periods of the day causes deflection of the gal- 

 vanometer which is proportional to that intensity. The 

 maximum deflection of the galvanometer employed is 

 attained in the course of 3 seconds after exposure. 



The third key, k 3 , is for the completion of a spark cir- 

 cuit which records the maximum galvanometric deflection 

 3 seconds after the exposure of the selenium cell. This key 

 actuates a sparking coil R, the vibrating interrupter of which 

 is not shown in the figure. The sparks, thus produced, 

 puncture the maximum deflection of the galvanometer index 

 on the moving sheet of paper. The details of this process 

 will be presently given. 



The successive closing and opening of the keys are made 

 automatically and in proper sequence by means of clock- 

 work, the whole process being repeated at intervals of 

 15 minutes. 



- The Galvanograph 



We now come to the difficult problem of the auto- 

 matic record of the galvanometer deflections. This might 

 be secured without great difficulty by means of photo- 

 graphy. A spot of light reflected from the galvanometer 

 mirror might be allowed to fall on a photographic plate which 



