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Prof. J. C. Bose. On Electromotive Wave [May 21, 



points. In order that a resultant effect may be exhibited in the 

 galvanometer, matters have to be so arranged that (1) the disturbance 

 may reach one point, say A and not B, and vice versd. This may be 

 accomplished by the method of block. Again, a resultant differential 

 action may be obtained even when the disturbance reaches both A 

 and B, if the electrical excitability of one point is relatively exalted 

 or depressed by physical or chemical means : we thus have two other 

 means of obtaining a resultant effect, (2) by the method of relative 

 exaltation, (3) by the method of relative depression. 



Method of Block. 



The electromotive effect described below may be obtained with all 

 metals. A piece of " tin " wire (an alloy of tin and lead used as 

 electric fuse) will be found to give very good results. A specimen of 

 wire 1 mm. in diameter, 10 cm. in length, is mounted in the apparatus. 

 (Fig. 1, b.) Two strips of cloth moistened with water or dilute salt 

 solution are securely tied round two points A and B. They hang 

 loose from EE' (non-polarising electrodes — Zn in ZnS0 4 solution), so 

 that there is no pull on the wire. Special precautions are taken so 

 that there is no variation of contact. If a sharp tap be given to the 

 side A, a transitory electrical current in response to the disturbance will 

 flow round the circuit, which under normal conditions will be found to 

 flow in the wire towards the more excited end A. Disturbance of B will 

 give rise to a reverse current. Eor quantitative measurement it is 

 necessary to have the intensity of stimulus maintained uniform or 

 increased or decreased in a definite manner. Instead of a tap, the 

 stimulus of torsional vibration is more satisfactory. By maintaining 

 the amplitude of vibration constant or increasing or decreasing the 

 amplitude, we may either keep the stimulus constant or increase or 

 decrease it in a definite manner. I shall first describe some of the 

 typical results which may be obtained with the simple " straight wire 

 form " of the apparatus. If worked with care it will give consistent 

 and satisfactory results. For quantitative measurements requiring 

 the greatest exactitude the "cell form," to be presently described, 

 will be found preferable. 



Recording Apparatus. — The galvanometer used is a sensitive dead- 

 beat D'Arsonval ; the period of complete vibration of suspended coil 

 under experimental conditions is 11 seconds. The records are taken 

 by means of a cylindrical modification of the response recorder described 

 in my previous paper,* or by means of photography. In the latter 

 method, a clockwork moves the photographic plate at a uniform rate 

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



* " On the Continuity of Effect of Light and Electric Eadiation on Matter,'*' 

 Eoy. Soc. Proc.,' vol. '70, p. 159. 



