100 CHAP. XIII. THE AUTOMATIC RADIOGRAPH 



the continued action of an electric current ; this is due to 

 polarisation caused by the current, which increases with its 

 strength and duration. But the effect of polarisation is 

 negligible if the current be feeble and of short duration. 

 Another difficulty which might possibly interfere with the 

 accuracy of the readings is the effect of daily variation of 

 temperature on the normal resistance of the selenium cell. 

 This effect may be eliminated by observing, at different 

 hours of the day, the difference in the resistance of the cell 

 (i) in the dark, and (2) after exposure to light. Finally, we 

 have to devise some means for the automatic record of the 

 galvanometric deflection under changing intensities of light. 



The Radiograph 



The difficulties enumerated above have been completely 

 overcome by the following devices : 



(a) A Wheatstone Bridge for balancing the electric 

 resistance of the selenium cell in darkness and its upset on 

 exposure to light ; 



(b) An arrangement of three electric keys which are 

 automatically put on and off in regular sequence and at 

 predetermined intervals ; 



(c) A Self-recording Galvanograph. 



The Wheatstone Bridge 



This is diagrammatically represented in b (fig. 29). 

 The resistance of the particular selenium cell S is 76,000 

 ohms in the dark. An approximately equal resistance is 

 placed in the second arm of the bridge. A rheostat, with 

 a large number of turns of fine wire with a sliding contact, 

 is used for the two variable arms of the bridge, diagram- 

 matically represented by a straight line. An approximate 

 balance is obtained when the sliding contact is in the middle ; 

 a slight movement to the right or to the left secures the 

 exact balance, when the galvanometer deflection is reduced 

 to zero. The balance is upset when the selenium cell is 



