in Conducting Sheets and Solid Bodies. 2 1 



If we neglect self-induction, we haA r e 



where % is a two-dimension potential function, finite at the 



origin, and equal to a constant plus — at the boundary, the 



determination of which is a well-known problem in electro- 

 statics. 



In the Phil. Mag. for 1880 Professor Guthrie and Mr. 0. V. 

 Boys give details of experiments on the couples experienced 

 by Arago's disks in a rotating magnetic field; and in a second 

 paper is given an account of the determination of the relative 

 resistances of polarizable liquids from the couples experienced 

 by the containing vessel in a uniform rapidly rotating mag- 

 netic field. It would be very interesting to determine the 

 absolute value of the couple for a copper sphere in a uniform 

 rotating field of known strength, and to determine the log. 

 decrement of its torsional oscillations due to induction in a 

 stationary field, and to deduce thence, by approximation from 

 the general formulae, two values for the specific resistance of 

 its material at the given temperature in absolute units, which 

 should be consistent with themselves and with the known, 

 value. For a uniform thin spherical copper shell the calcula- 

 tion would be very simple, and at the same time the tempera- 

 ture conditions uniform. No absolute quantitative experi- 

 mental verification appears to have yet been made for induc- 

 tion-currents in continuous media. 



Since writing the above, I have found that H. Hertz has 

 given solutions of problems of rotation to a considerable 

 extent identical with those discussed here (Inaugural Dis- 

 sertation, Berlin, 1880, pp. 93). I have not yet been able to 

 procure his paper ; but the results, so far as indicated in a 

 notice in Phil. Mag. Dec. 1880, agree with the above. 



VIII. In the case of the Earth rotating round its axis we 

 have, approximately, 



_10 9 2tt 



ac °~~ 2^' 24x60x60' 



therefore, in a uniform field (n=l, s=l)j 



3R 



p= 



A7raco 



2R ... 



= Jq5 approximately, 



