6 COMPARATIVE ELECTRO-PHYSIOLOGY 



excitation of the A end of the wire, by imparting a torsional 

 vibration, the molecular disturbance being prevented from 

 reaching the B end by the intervening block. Using this 

 method of experiment, I have obtained with different sub- 

 stances two different types of response namely, positive 

 and negative. In the positive, the responsive current flows 

 through the wire from the unexcited to the excited, or 

 towards the excited that is to say, the excited point 

 becomes galvanometrically positive. Responses of this kind 

 are given by tin, zinc, platinum, and other metals. In fig. 7 

 is seen a uniform series of such responses 

 to uniform stimulus. The intensity of the 

 response, moreover, does not appear to 

 depend on the chemical activity of the 

 substance. For the response of the chemi- 

 cally inactive tin is much stronger than 

 that of the active zinc. The very inactive 

 platinum is also found to give a fairly 

 stron g response, although the electrolytic 

 contacts are made with pure water. 



The electro-motive response may also be obtained by other 

 modes of molecular excitation. Thus, instead of torsional, 

 we may use longitudinal vibration. A metallic rod of brass, 

 A C, is clamped in the middle. A thin copper wire is led 

 sideways from the clamp and connected with a piece of 

 brass, B. A and B are connected with a galvanometer by 

 means of non-polarisable electrodes. If now the C end of 

 the rod be rubbed with resined cloth, A may be thrown into 

 longitudinal vibration, B being little affected by this. It is 

 here interesting to observe the concomitance of the electrical 

 response with the sonorous tesponse of the rod, and the 

 dying of the electrical response with the waning of the musical 

 note. The stronger the molecular vibration, the stronger the 

 sound, and also the stronger the response. The direction of 

 the responsive current in the metal is from the less excited B 

 to the more excited A. 



We found under the method of conductivity variation 



