1901.] 



between Radiation and Mechanical Strains. 



175 



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 

 effect 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 sensi- 

 tive 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), 

 Ttept moderately stretched by springs CC. 

 A long handle, A, provided with a pointer, 

 could be attached either to E or E', and 

 hy its means either of the wires could be 

 twisted. The angle of the twist is meas- 

 ured 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 f " 



molecular condition. Fig. 11.— The Strain Cell. 



N 2 



