198 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



method for obtaining a continuous record for the whole day. 

 The resistance of the selenium cell undergoes a change due 

 to polarisation under the continued action of the electric 

 current ; but this can be rendered neghgible by maintaining 

 a feeble current for a very short time. The variation of 

 temperature at different periods may also affect the resist- 

 ance of the selenium cell. This will be shown to be very 

 slight and practically neghgible. The most difficult problem 

 is the automatic record of the galvanometric deflection 

 under changing intensity of light. 



The complete Radiograph consists of : 



1. The Wheatstone Bridge for balancing the electric 



resistance of the selenium cell in the dark, the 

 balance being upset on exposure to light. 



2. The arrangement of three electric keys which are 



automatically put on and off in regular sequence 

 and at predetermined intervals. 



3. The Self-Recording Galvanograph. 



Tlie Wheatstone Bridge. — This is diagrammatically repre- 

 sented in B (fig. 115). The resistance of the particular 

 selenium cell S is 76,000 ohms in the dark. An approxi- 

 mately equal resistance is placed in the second arm of the 

 bridge. A rheostat having a large number of turns of fine 

 wire with a sliding contact is used for the two variable arms 

 of the bridge, diagrammatically represented by a straight 

 line. An approximate balance is obtained when the shding 

 contact is in the middle ; a shght movement to the right 

 or to the left secures the exact balance, when the galvano- 

 meter deflection is reduced to zero. The balance is upset 

 when the selenium cell is 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^, after which the selenium cell is exposed 

 to light by an automatic electro-magnetic shutter. The 

 deflection of the galvanometer is recorded on a piece of 



