104 CHAP. XIII. THE AUTOMATIC RADIOGRAPH 



is thus no resultant kick, and the index remains undisturbed. 

 The sparking, as previously stated, takes place 3 seconds 

 after exposure of the selenium cell to light, by which time 



Fig. 30. Radiogram of Variation of Intensity of Light from the 

 sky during 12 hours in winter 



The upper record shows the variation on a bright day, the 

 maximum intensity being attained at 12 noon. The lower 

 record exhibits irregular variation on a cloudy day. The 

 horizontal record above the base-line shows that the electric 

 resistance of the selenium cell is but slightly affected by 

 variation of temperature. Successive thin dots at 15 minutes' 

 interval, thick dots at intervals of an hour. 



the deflection has reached its maximum. The record thus 

 consists of successive dots at intervals of 15 minutes, the dots 

 representing the maximum deflections of the galvanometer 

 corresponding to the intensity of light. 



