AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
133 
aid in investigations on the liability of wrought-iron axles to fracture produced by 
sudden changes of the temperature of the air. 
Again, it will he observed from Experiment LYI. that shortly after the iron has 
been heated and then cooled there is less elasticity than when a considerable rest has 
been allowed; and in fact we have in this case exactly the same kind of restitution of 
elasticity in iron as we have seen takes place after the wire has experienced mechanical 
extension. With nickel the increase of elasticity produced by rest after cooling is still 
more remarkable. Equally remarkable also is the temporary change of elasticity 
produced in nickel; and a comparison of the loss of elasticity produced by raising the 
temperature to 100° C. with the change of susceptibility to alteration of resistance 
from change of stress as shown in Table XXIII. affords a still further proof that stress 
and strain act in the contrary direction as far as electrical conductivity is concerned. 
It will also be noticed in Table XXV. that the alteration of susceptibility to change 
of electrical resistance from change of stress is greater in iron in proportion to the 
alteration of elasticity when the temperature is raised to 100° C., and that the converse 
is the case with copper, so that there is a greater or less alteration of specific resistance 
for the loaded wire than for the unloaded, caused by rise of temperature to 100° C., 
according as the metal is iron or copper; and we have seen that a similar state of 
things occurs where the strain is that left after the removal of the stress. 
‘ 
The Alteration of Electrical Conductivity produced by Magnetization. 
History of the subject and description of apparatus. 
This subject has, in the case of iron, received the attention of several observers, who 
have in some instances differed not only as regards the amount but also as regards 
the nature of the change produced by magnetization on the electrical resistance. Sir 
! W. Thomson, in 1856,* * * § was I believe the first to show that magnetization affected the 
electrical conductivity of iron and steel, longitudinal magnetization causing in these 
metals increase, and transverse magnetization decrease of resistance, and this with 
steel was found to be the case whether the metal was hard or soft. Shortly after¬ 
wards experimenting on nickel, Thomson foundt with this metal also an alteration of 
resistance similar in kind to that of iron, but greater in amount for the same magnetiz¬ 
ing force. With brass, on the contrary, he failed to detect any change whatever. 
Subsequently Beetz,| Chwolson,§ and myself || pursued similar investigations. 
* Phil. Trans., “Electrodynamic Qualities of Metals,” Part IV. 
t Proc. Roy. Soc., vol. viii., 1857. 
X Pogg. Ann., vol. cxxviii., p. 202 (1866). 
§ Carl’s Rep., vol. xiii., p. 232 (1877). 
|| Proc. Roy, Soc., June 17, 1875, vol. xxiii., p. 533. 
