460 Prof. J. J. Thomson on the Discharge of Electricity 



to the differences between their specific resistances. The 

 first experiment I tried with this object was to cover the iron 

 rod with thin sheet platinum, such as is used for Grove cells. 

 As the resistance of platinum is not very different from that 

 of iron, if the effect depended merely upon the resistance, 

 slipping a thin tube of platinum over the iron ought to make 

 very little difference. I found, however, that when the 

 platinum was placed over the iron, all the peculiar effects pro- 

 duced by the latter were absent, thus showing that the effect 

 is not due to the resistance of the iron. It then occurred to me 

 that I might test the same thing in another way by magnetizing 

 the iron to saturation, for in this state //, is nearly unity ; thus 

 if the result depended mainly on the magnetic properties of 

 the iron it ought to diminish when the latter is strongly 

 magnetized. I accordingly tried an experiment in which 

 the iron in the coil B was placed between the poles of 

 a powerful electromagnet. When the magnet was " off" the 

 iron almost stopped the discharge in A; when it was " on" the 

 discharge became brighter, not indeed so bright as if the iron 

 were away altogether, but still unmistakably brighter than 

 when it was unmagnetized. This experiment, I think, proves 

 that iron retains its magnetic properties when exposed to 

 these rapidly alternating forces. 



Another result worthy of remark is that though a brass 

 rod or tube inserted in B does not stop the discharge in A, 

 yet if a piece of glass tubing of the same dimensions is coated 

 with Dutch metal, or if it has a thin film of silver deposited 

 upon it, it will stop the discharge very decidedly. We are 

 thus led to the somewhat unexpected result that a thin layer 

 of metal when exposed to these very rapid electrical vibra- 

 tions may absorb more heat than a thick one. I find, on 

 calculating the heating-effect in slabs of various thicknesses, 

 that there is a thickness for wdiich the heat absorbed is a 

 maximum. Instead of taking the case of a cylinder, for 

 which the analysis is somewhat long, I will take the case 

 (which is physically almost equivalent to it) of a plain elec- 

 trical wave incident normally upon a plane sheet of metal. 



Let the electromotive intensity E in the incident wave be 

 represented by the real part of the equation 



the axis of x being at right angles to the plate, and the origin 

 on the upper surface of the plate. The electromotive inten- 

 sity Ej in the wave reflected from the plate may be represented 

 by the equation 



E 1 =be L( P t ~ ax \ 



