200 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



is shown in the diagram immediately above the dark box. 

 In reahty 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 thus 

 from a definite area of the sky. The intensity of light 

 from the sky at different periods of the day causes deflection 

 of the galvanometer 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, Kg, is for the completion of the spark- 

 circuit for record of 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 spark thus produced 

 punctures the maximum deflection of the galvanometer 

 index on a moving piece of paper attached to the plate M. 

 The successive closure 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. — Now comes the most difficult 

 problem — the automatic record of the galvanometer de- 

 flections. A record may be secured without great difficulty 

 by means of photography. A spot of light reflected from 

 the galvanometer mirror may be allowed to fall on a photo- 

 graphic plate which descends at a uniform rate by clock- 

 work. This, however, entails the use of a dark room and 

 subsequent development of the plate. This trouble was 

 avoided by the device of direct record of the galvanometer 

 deflection by means of electric sparks. 



A sparking method has been previously employed in 

 which the deflected index of the galvanometer in connection 

 with one electrode of an induction coil leaves a spark-record 

 on a moving piece of paper. Several difficulties are, how- 

 ever, encountered in the employment of this method with 

 a highly sensitive galvanometer. There is a liability of 

 leakage of the high-tension current into the galvanometer 



