22 RESPONSE IN THE LIVING AND NON-LIVING 



Photographic recorder. Or the records may be 

 made photographically. A clockwork arrangement 

 moves a photographic plate at a known uniform rate, 

 and a curve is traced on the plate by the moving spot of 

 light. All the records that will be given are accurate 

 reproductions of those obtained by one of these two 

 methods. Photographic records are reproduced in 

 white against a black background. 



Compensator. As the responses are on variation of 

 current of injury, and as the current of injury may be 



strong, and throw the 

 spot of light beyond 

 the recording surface, 

 a potentiometer ba- 

 lancing arrangement 

 may be used (fig. 9), 

 by which the P.D.due 



FIG. 9.-THE COMPENSATOR ^ in j urv ' ig exactly 



A B is a stretched wire with added resistances R 



and B'. S is a storage cell. When the key Compensated : E.M. 

 K is turned to the right one scale division = 



001 volt, when turned to the left one scale variations produced 

 division = -01 volt. P is the plant. 



by stimulus are then 



taken in the usual manner. This compensating arrange- 

 ment is also helpful, as has been said before, for 

 calibrating the E.M. value of the deflection. 



Means of graduating the intensity of stimulus. One of 

 the necessities in connection with quantitative measure- 

 ments is to be certain that the intensity of successive 

 stimuli is (1) constant, or (2) capable of gradual increase 

 by known amounts. No two taps given by the hand 

 can be made exactly alike. I have therefore devised 

 the two following methods of stimulation, which have 

 been found to act satisfactorily. 



