166 Mr. C. S. Whitehead on the Effect of a Solid 



where E^ is the maximum value of the magnetic induction 



normal to the surface, 

 Ta the maximum value of the induction tangential to the 



surface, 

 / the radius of the inducing circuit, 

 d the distance of the inducing circuit from the surface of 



the plate, 

 p the distance of the point from the axis, 

 7 the maximum current in the inducing circuit, 

 fi the magnetic permeability of the plate, 

 a its specific resistance, 

 p = 2-7T x freq uency , 



From these equations, and with the values of p, fi, and <r 

 given in the paper, the value of E^ for a sea-water plate is 

 44 times as great as it would be for an iron plate, and more 

 than 3000 times as great as it would be for a copper plate. 



It is also seen that for the purpose of Induction Telegraphy 

 the receiving coil should have its plane vertical and not 

 horizontal to get the best effect ; firstly because d must in 

 practice be small compared with p, and hence Rj is small 

 compared with T^ ; and secondly because R d varies inversely 

 as p while T^ varies inversely as p i . 



Let P, Q, E be the components of the electromotive in- 

 tensity, 



a, b, c the components of the magnetic induction, 



a the specific resistance of the sphere, 



p, the magnetic permeabilhVy of the sphere. 



Let P, Q, E all vary as e ipt , where p = 2irx frequency and 



Neglecting the polarization-current, P, Q, E (in the sphere) 

 satisfy 



„-,-, 4-7T/X dP 



v p= 



Let 



a dt ' 

 &c. = &c. 



where _/2Trnp\* 



.'. V 2 P + £ 2 P = (1) 



with similar equations for Q and E. We also have 

 d? dQ dR A 

 dx dy dz 



