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 versá. 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 ZnSO4 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. For 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,[1] 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 Radiation on Matter," Roy. Soc. Proc.,' vol. 70, p. 159.
