626 Professor Jagadis Chunder Bose [May 10, 



With tin wire under normal conditions, the current through the 

 wire is always from the unexcited to the excited end, and from the 

 excited to the unexcited through the galvanometer.* But just as in 

 living substances we find two opposite kinds of response (e.g. nerve 

 giving negative and retina positive variation), so also the responses 

 given by some inorganic substances are of opposite signs to that of 

 tin. For example, silver sometimes, especially in cold weather, passes 

 into a peculiar molecular condition in which it gives the reverse re- 

 sponse to that of tin, the " action current " in the wire being from 

 the excited to the unexcited. An interesting transition from one 

 class to the other is sometimes found in the behaviour of lead. 

 Under feeble stimulus the current is away from the stimulated, and 

 under stronger, towards that end. The majority of metals, however, 

 behave like tin. 



This simple form of experiment with metallic wire has been de- 

 vised for the special purpose of bringing the essential points out 

 clearly. But it labours under certain defects. Unless carefully carried 

 out, there may be shifting of contacts ; there may be variations of 

 resistance by the evaporation of the liquid contacts ; and quantitative 

 measurements also are rendered difficult, for want of some means of 

 graduating the intensity of stimulus. I will now describe a perfect 

 form of apparatus for exhibiting the electrical response of metallic 

 wires to mechanical stimulus, in which all these difficulties have 

 been completely overcome. 



In the typical experiment (Fig. 8), instead of 



Experimental making the galvanometer connection through elec- 



Modifications. trolytic contacts we may cut A B into two (6, 



Fig. 9), and place the galvanometer in the gap, 



connecting A B directly by electrolyte. 



This leads to c, Fig. 9, where A and B are held parallel to each 

 other in an electrolytic bath (water).f Mechanical vibration may 

 now be applied to A without affecting B, and vice versa. 



The actual apparatus, of which this is a diagrammatic representa- 

 tion, is seen in d, Fig. 9. 



Two pieces, from the same specimen of wire, are clamped sepa- 

 rately at their lower ends by means of ebonite screws, in an L-shaped 

 piece of ebonite. The wires are fixed at their upper ends to two 

 electrodes (leading to the galvanometer), and kept moderately and 

 uniformly stretched by spiral springs. The handle, by which a tor- 

 sional vibration is imparted to the wire, may be slipped over either 

 electrode. The amplitude of vibration is measured by means of a 

 graduated circle, not shown in the figure. 



* The galvanometer in the above arrangement is interposed, as it were, in 

 the electrolytic part of a voltaic cell. The portion of tin wire under excitement 

 becomes zincoid. I mention this, as some misunderstandings and wrong in- 

 ferences have arisen from not distinguishing between the direction of the current 

 in the electrolytic part and that in the rest of the circuit. 



t In all the experiments hereafter described the electrolyte is water unless 

 the contrary be stated. 



