﻿60 Dr. C. V. Burton on the 



impressed upon the system from without. This condition, 

 combined with (i.), will evidently give us Ohm's Law. 



Now the forces actually present and tending to modify the 

 heat-movements are of two kinds : electromagnetic and 

 electrostatic. 



(a) Electromagnetic Forces. — The passage of a current 

 through a conductor gives rise to a magnetic field, which 

 may or may not appreciably affect the conductivity. The 

 thin iron wire used by Prof. Chrystal was *0021 cm. in 

 radius, and the greatest value of the magnetic force due to a 

 current of 1 ampere per square centimetre of cross section 

 would be in absolute measure about -0013 (at the surface of 

 the wire), the square of the greatest magnetic force being 

 thus about -0000017. The average value of (magnetic 

 force) 2 over the cross section of the wire would be half of 

 this, or -00000085; that is, about -0000039 of the square of 

 the terrestrial " total force " in these parts. 



Now Lord Kelvin found * that the change of resistance 

 due to transverse magnetization of an iron plate by a powerful 

 Ruhmkorff electromagnet was only just decided enough to be 

 distinctly appreciated with the apparatus which he employed, 

 and we may therefore conclude that in Prof. Chrystal's iron 

 wire no perceptible change of resistance could have been 

 produced by the magnetic field of the current. In other 

 metals the effect must be still more insignificant. 



On the other hand, the longitudinal magnetization of an 

 iron wire perceptibly increases its electrical resistance, so 

 that it would be easy to construct a simple conductor whose 

 resistance at a given temperature was a function of the 

 current-strength. For let a flat bobbin be wound with iron 

 wire, so that each turn has the form of an elongated rectangle, 

 and then let a further quantity of iron wire be wound in a 

 similar circuit embracing the first. Finally let the coils be 

 joined in series with a source of E.M.F. When a current 

 is sent through the circuit, each coil will magnetize longi- 

 tudinally some parts of the wire of the other coil, and so, for 

 a given temperature of the wire, the resistance will increase 

 with the current. 



(b) Electrostatic Forces.- — Let us attempt to calculate the 

 electrostatic energy per cubic centimetre which a mass of iron 

 possesses in virtue of a current flowing through it with a 

 " density "'of 1 ampere per cm. 2 To do this we must assume 

 some value for the specific inductive capacity of iron f, and 



* Phil. Trans. 1856, especially pp. 747-749. 



t In electrostatic measurements conductors appear to have an infinite 

 specific inductive capacity ; but here, where the potential really varies 

 from point to point through the metal, it is the true (finite) specific 

 inductive capacity which concerns us. 



