96 RESPONSE IN THE LIVING AND NON-LIVING 



processes of annealing or stretching, the effects obtained 

 on subjecting A or B to any given stimulus are always 

 equal (fig. 56). 



Usually I interpose an external resistance varying 

 from one to five megohms according to the sensitive- 

 ness of the wire. The resistance of the electrolyte in 

 the cell is thus relatively small, and the galvanometer 

 deflections are proportional to the E.M. variations. It 

 is always advisable to have a high external resistance, 

 as by this means one is not only able to keep the 

 deflections within the scale, but one is not troubled by 

 slight accidental disturbances. 



Graduation of intensity of stimulus. If now a rapid 

 torsional vibration be given to A or B, an E.M. variation 



will be induced. If the 

 amplitude of vibration be 

 kept constant,- successive 

 responses in substances 

 which, like tin, show no 

 fatigue will be found to 

 be absolutely identical. But 

 as 'the amplitude of vibra- 

 tion' is increased, response 

 will also become enhanced 

 (see Chap. XV). 



Amplitude of vibration 

 is measured by means of the 

 graduated circle (fig. 57). A 

 projecting index, in connection with the vibration-head, 

 plays between fixed and sliding stops (s and s'), one 

 at the zero point of the scale, and the other movable. 



FIG. 57. TOP VIEW OF THE VIBRA- 

 TION CELL 



The amplitude of vibration is deter- 

 mined by means of movable stops 

 S S', fixed to the edge of the gra- 

 duated circle G. The index arm 

 I plays between the stops. (The 

 second index arm, connected with 

 B, and the second circle are not 

 shown.) 



