AND RESISTANCE OF NICKEL WIRE AT HIGH TEMPERATURES. om 
question here raised would probably repay further experimental investigation ; and it 
obviously suggests a new method for studying magnetic screening. 
6. COMPARISON WITH OTHER PECULIARITIES OF NICKEL Ar 200° C.—That some 
kind of peculiarity should occur at about this temperature was not unexpected. 
It was indeed with the expectation of getting some such effect that these experiments 
were originally planned more than a dozen years. It was my good fortune as 
an undergraduate to assist the late Professor Tarr in the thermoelectric investiga- 
tions which occupied his attention during the years 1872-4.* Probably the 
most remarkable results established by these investigations were those in connection 
with iron and nickel. The thermoelectric lines for all metals save iron and nickel are 
approximately straight through great ranges of temperature. Their inclinations in the 
properly constructed thermoelectric diagram give the Thomson Effects in the corre- 
sponding metals. In every case of a pure metal except those mentioned, the Thomson 
Effect retains the same sign throughout. In the case of iron and nickel, however, it 
changes sign—at a dull red heat in the case of iron, and at about 180°-200° in the 
case of nickel. But the nature of the phenomenon is the same in both. The Thomson 
Effect, which is negative at ordinary temperatures, becomes positive at higher 
temperatures ; and finally, when the temperature is raised still higher, negative again. 
The second change of sign occurs in each case at the temperature at which the metal 
ceases to be strongly paramagnetic. In the case of iron, another phenomenon is known 
to occur at the temperature of dull red, namely, the sudden expansion during cooling 
discovered by GorE, and the accompanying reglow discovered by Barrerr. No 
similar effect has been observed in the case of nickel, possibly not because it does not 
exist, but because the temperature is too low to admit of a visible ‘reglow.’ In any 
case these phenomena point to a curious molecular change occurring both in iron and 
in nickel at a temperature well below that at which the magnetic permeability becomes 
unity. 
Mention has already been made as to the rather curious manner in which the 
resistance of nickel changes with temperature. In my paper on the electric resistance 
of nickel at high temperatures, referred to above, this peculiarity is established, and 
the difficulty in working at high enough temperatures prevented me establishing the 
existence of the same peculiarity in iron, although there was indication of its existence. 
This, however, was done shortly afterwards by W. Koutrauscu. The peculiarity in 
the case of nickel is shown by the interpolation formule given above, p. 41. The 
rate of increase of resistance with temperature undergoes a sudden increase at a 
temperature of about 180°-200° C., and then diminishes as abruptly again at about 
400° C. Once again, then, we have another set of phenomena indicating a peculiar 
molecular change in nickel at 200° as well as at 400°. 
In the present investigation the relation that is being studied involves the measure- 
* See Tart, Trans, Roy. Soc. Hdin., vol. xxvii. pp. 125-140; also Scientific Papers, vol. i. pp. 218-233. 
TRANS, ROY. SOC. EDIN., VOL. XLI. PART I. (NO. 3). 9 
