330 Mr. J. Buchanan on the Theory of 



field due to the moving bodies, must leave the magnetic field 

 unaltered. But this electrification is precisely what would be 

 acquired if the glass tube were (as under ordinary experi- 

 mental conditions it is) slightly conducting. Such a tube 

 will not destroy the external magnetic field. 



11. If we suppose a coil of wire to lie outside the tube, no 

 lines of force pass through it, and no current will be induced 

 in it by a variation in the velocity of the charged body or of 

 the charge that it carries. Regarding the tube itself as an 

 aggregate of wires parallel to the axis, the same remark 

 applies, and there will be no electromotive force of induction 

 tending to cause a current to flow along it. 



12. The equation in the case of a rotating charged disk is 

 different, and cannot be integrated out; but the main dif- 

 ficulty lies in the discussion of the term corresponding 

 to t . It seems to be impossible to prove that it is unim- 

 portant for our purpose, and it is quite impossible to evaluate 

 it without making assumptions as to the equations satisfied 

 by the vectors of the electromagnetic field at the surface of 

 moving conductors. We cannot say with certainty whether a 

 conducting envelope can, as in the case of § 9, by screening off 

 the electric force reduce the magnetic force also to zero, or 

 whether the surface of the wire in a coil can exert such a 

 screening effect (either partial or total) on the substance of 

 the wire. If a total or considerable partial screening cannot 

 be shown to be theoretically impossible, Cremieu's experi- 

 ment * does not afford any decisive evidence of the want of 

 truth of the hitherto accepted equations of the electromagnetic 

 field. 



XXIX. A Contribution to the Theory of Magnetic Induction in 

 Iron and other Metals. By John Buchanan, B.Sc.f 



IN connexion with his exquisite work relating to the 

 diffusion of heat, Fourier has given us a number of 

 functions with most wonderful properties. To anyone who 

 has made himself acquainted with the graphs which express 

 some of these functions, many of the curves given by 

 Prof. Ewing in his classical researches on magnetic induction 

 must have seemed oddly familiar. 



It is well known that magnetic problems can be trans- 

 formed into problems in electricity and in heat (Clerk 

 Maxwell's Elect, and Mag. 2nd ed. vol. ii. par. 428 et seq,) . 



The object of this paper, however, is to show — what does 



* V. Cremieu, Comptes Rendus, cxxx. p. 1544 (1900) 

 t Communicated by the Author. 



