sible to radiation is very much enlarged. Moreover, the resistance offered to the particles is not due to the individual solid lumps, but to the resistance of surface layer. It is precisely the surface layers that are affected by radiation, and hence the marked variation of resistance.
When the particles become continuous, the radiation can only affect the extremely thin layer of molecules on the surface, the mass in the interior being shielded by the outer conducting sheet; the molecular changes produced on the surface layer do not affect to any appreciable extent the conductivity of the mass.
For detection of strain effect in continuous solids the method of electromotive variation is more suitable. We have seen that light causes a P.D. between the acted and unacted plate. We shall employ this method to find out whether mechanical disturbance gives rise to a similar electromotive variation between the acted and unacted plate.
1. The Strain Cell.
For the purpose of the experiment, I made a voltaic element composed of two pieces of the same metal wire, W.W′, cut from the same length. These are fixed parallel to each other in an L-shaped piece of ebonite, see fig. 11. The upper ends of the wires are in connection with the electrodes EE′, which lead to a very sensitive dead-beat galvanometer of D'Arsonval type. The wires at their lower ends are fixed to the ebonite piece by means of ebonite screws SS′. The upper ends, as has been said, are fixed to metallic rods EE′ (which also serve as the electrodes), kept moderately stretched by springs CC′. A long handle, A, provided with a pointer, could be attached either to E or E′, and, by its means either of the wires could be twisted. The angle of the twist is measured with the help of a graduated circle, not shown in the figure.
If a cell be made of two clean wires cut from the same piece, with water as the electrolyte, there should theoretically be no P.D. between the two. But in practice there is found a small P.D. between the wires, owing to small difference in the molecular condition.
