172 



AFTER-EFFECT. 



[CH. VII 



(203) After-effects recorded on a rotating surface. 



The movements described at the end of exp. 202 may 

 be made to record themselves in the following way. 



FIG. 31. Exp. 203. 



The writer (fig. 31) is made of two light pieces of wood, 

 the horizontal piece ends in a pointed piece of platinum 

 foil, while the vertical piece ends in a loop of cotton by 

 which it is attached to the shoot. By twisting the loop 

 it is easy to make the foil press lightly against the 

 smoked paper of a revolving drum 1 ; the T piece being 

 light (about 0'2 gram) it does not interfere with the 

 geotropic action. In a typical experiment of this sort the 

 primary geotropic rise lasted 4 hours, the after-effect 

 2 hours. Then came 2 hours in which the index was 

 stationary ; finally it rose once more, beginning very 

 slowly. 



1 Or unsmoked paper may be used with a Dittmar writer. 



