92 RESPONSE IN THE LIVING AND NON-LIVING 



completely removed by the adoption of the following 

 experimental modification. In the simple arrangement 

 for qualitative demonstration of response in metals 

 previously described, successive experiments will not 

 give results which are strictly comparable (1) unless 

 the resistance of the circuit be maintained constant. 

 This would necessitate the adoption of some plan for 

 keeping the electrolytic contacts at A and B absolutely 

 invariable. There should then be no chance of any 

 shifting or variation of contact. (2) There must also 

 be some means of applying successive stimuli of equal 

 intensity. (3) And for certain further experiments 

 it will be necessary to have some way of gradually 

 increasing or decreasing the stimuli in a definite 

 manner. 



Modification of the block method. By consideration 

 of the following experimental modifications of the block 

 method (fig. 55), it will be found easy to construct 

 a perfected form of apparatus, in which all these con- 

 ditions are fully met. The essentials to be kept in 

 mind were the introduction of a complete block midway 

 in the wire, so that the disturbance of one half should 

 be prevented from reaching the other, and the making 

 of a perfect electrolytic contact for the electrodes 

 leading to the galvanometer. 



Starting from the simple arrangement previously de- 

 scribed where a straight wire is clamped in the middle 

 (fig. 55, a), we next arrive at (b). Here the wire 

 A B is placed in a U tube and clamped in the middle 

 by a tightly fitting cork. Melted paraffin wax is poured 

 to a certain depth in the bend of the tube. The two 



