168 



Mr. P. Griess on some 



[Feb. 15, 



currents in an infinite plane sheet has been solved, taking into account the 

 mutual induction of these currents, so as to make the solution applicable 

 to a sheet'of any degree of conductivity. 



The statement in equation (10), that the motion of a magnetic system 

 does not produce differences of potential in the infinite sheet, may appear 

 somewhat strange, since we know that currents maybe collected by electrodes 

 touching the sheet at different points. These currents, however, depend 

 entirely on the inductive action on the part of the circuit not included in 

 the sheet ; for if the whole circuit lies in the plane of the sheet, but is so 

 arranged as not to interfere with the uniform conductivity of the sheet, there 

 will be no difference of potential in any part of the circuit. This is pointed 

 out by Felici, who shows that when the currents are induced by the instan- 

 taneous magnetization of a magnet, these currents are not accompanied with 

 differences of potential in different parts of the sheet. 



When the sheet is itself in motion, it appears, from art. 600 of my 

 treatise ( On Electricity and Magnetism,' that the electric potential of any 

 point, as measured by means of the electrodes of a fixed circuit, is 



B# By B^ 



where gj' ^ are the components of the velocity of the part 



of the sheet to which the electrode is applied. 



In the case of a sheet revolving with velocity ou about the axis of z, this 

 becomes 



Note 2. — The velocity R for a copper plate of best quality 1 millimetre 

 in thickness is about 25 metres per second. Hence it is only for very 

 small velocities of the apparatus that we can obtain any approximation to 

 the true result by neglecting the mutual induction of the currents. — 

 Feb. 13. 



II. " On some Derivatives of Uramidobenzoic Acid." By P. Griess, 

 F.R.S. Eeceived January 15, 1872. 



This acid, of which I gave a short description some time ago*, has 

 the composition C s H 8 N 2 3 . I obtained it in the first instance from 

 the basic compound C 10 H 12 N 2 3 , which is one of the products of the ac- 

 tion of cyanogen on an alcoholic solution of amidobenzoic acid. Its 



* Zeitsch. f. Chem. 1868, p. 389. 



