APPENDIX 



159 



in a photographic carrier suspended by a thread from a wheel revolv- 

 ing by clockwork, descends vertically. The deflections of the 

 galvanometer spot are recorded laterally upon the line of the 

 horizontal slit. An electric bell gives warning when the plate has 

 completed its descent. If necessary, a chronograph, and a signal 

 to mark the beginning and close of excitation, are easily added 

 when required. 



Before. During. After. 



looo voit. 



FiG. 61. Response of an oxydised copper plate, illuminated 

 for seven seconds at one minute intervals. A prolonged illumination 

 was made in the middle of the series to see whether any sign of ex- 

 penditure would be elicited. (The first deflection by T ^ volt 

 turned into circuit exhibits signs of ordinary polarisation.) 



Before. ILLumln. 



After. 



FiG. 62. Similar observation on a chlorinated silver plate. In 

 consequence of prolonged illumination there is well-marked evidence 

 of alteration. 



Speed of registration. By reason of the inertia of the suspended 

 magnets and mirror of the galvanometer, we must content ourselves 

 with registering phenomena that are, comparatively speaking, pro- 

 longed, or repeated at regular intervals. The method is not adapted 

 to record phenomena that require a speed of recording surface 

 greater than 5 mm. per second. It is best adapted to the recording 

 of phenomena of long duration, or to reactions that are repeated 



