K. Morris (_Phil. Mag._
September 1897, p. 213) observed the remarkable alteration that takes
place in the iron resistance temperature curve in the neighbourhood of
780 deg. C. At that temperature the direction of the curvature of the
curve changes so that it becomes convex upwards instead of convex
downwards, and in addition the value of the temperature coefficient
undergoes a great reduction. The mean temperature coefficient of iron
in the neighbourhood of 0 deg. C. is 0.0057; at 765 deg. C. it rises
to a maximum value 0.0204; but at 1000 deg. C. it falls again to a
lower value, 0.00244. A similar rise to a maximum value and subsequent
fall are also noted in the case of the specific heat of iron. The
changes in the curvature of the resistivity curves are undoubtedly
connected with the molecular changes that occur in the magnetic metals
at their critical temperatures.

A fact of considerable interest in connexion with resistivity is the
influence exerted by a strong magnetic field in the case of some
metals, notably bismuth. It was discovered by A. Righi and confirmed
by S. A. Leduc (_Journ. de Phys._ 1886, 5, p. 116, and 1887, 6, p.
189) that if a pure bismuth wire is placed in a magnetic field
transversely to the direction of the magnetic field, its resistance is
considerably increased. This increase is greatly affected by the
temperature of the metal (Dewar and Fleming, _Proc. Roy. Soc._ 1897,
60, p. 427). The temperature coefficient of pure copper is an
important constant, and its value as determined by Messrs Clark, Forde
and Taylor in terms of Fahrenheit temperature is

[rho]t = [rho]32 {1 + 0.0023708(t - 32) + 0.0000034548(t - 32)^2}.

_Time Effects._--In the case of dielectric conductors, commonly called
insulators, such as indiarubber, guttapercha, glass and mica, the
electric resistivity is not only a function of the temperature but also
of the time during which the electromotive force employed to measure it
is imposed. Thus if an indiarubber-covered cable is immersed in water
and the resistance of the dielectric between the copper conductor and
the water measured by ascertaining the current which can be caused to
flow through it by an electromotive force, this current is found to vary
very rapidly with the time during which the electromotive force is
applied. Apart from the small initial effect due to the electrostatic
capacity of the cable, the applicatio