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Proceedings of the Royal Society 
to which the solution of the general linear differential equation of 
the second order can always be reduced. 
(c) The Electric Conductivity of Nickel. By C. Michie Smith 
and J. G-ordon MacGregor. 
Pure nickel foil, obtained in Paris by Dr Andrews, was cut into 
a spiral about 20 inches long, and it was on this spiral that all the 
following experiments were made. During the month of November 
1875 a large number of experiments were made as to its thermo- 
electric properties, and these were found to be almost identical with 
that of the specimen from observations on which the line was laid 
down on the “ thermo-electric diagram.” (Trans. R.S.E., 1872-3.) 
This line, it will be remembered, is a peculiar one, and is very similar 
to that of iron, with this difference, that the peculiar changes take 
place at much lower temperatures in nickel than in iron. Having 
thus finally determined the position of the line in the thermo-electric 
diagram, we were anxious to discover whether, like iron, it exhibited 
other peculiarities about the same temperature, and for this end we 
made the following experiments on the electric conductivity at dif- 
ferent temperatures. The method of observation was as follows : — 
To the two ends of the nickel spiral stout copper wires were 
soldered, and the whole was carefully fastened together in such a 
way that no two coils of the spiral could touch each other. Side 
by side with this nickel spiral was placed a similar spiral of soft 
platinum wire of approximately equal resistance. This platinum 
was part of a wire the electric conductivity of which had been 
formerly carefully tested, and had been found to obey very strictly 
the law of being proportional to the absolute temperature. These 
spirals were then placed in a large pot of oil, care being taken that 
they hung quite free from the sides of the pot, and the ends of the 
thick copper wires were led to the pools of a mercury commutator, 
so arranged that either the nickel or platinum could be made to 
form one of the arms of a Wheatstone’s bridge, in connection with a 
very delicate Thomson’s dead-beat mirror galvanometer. In 
making the observations the oil was heated by a powerful Bunsen 
burner, and constantly stirred. By this means it was found per- 
fectly practicable to keep the oil sensibly at the same temperature 
during the time necessary to find the resistance of the two wires 
