of Edinburgh, Session 1879-80. 
709 
increases, the high note comes in again ; then the fizzing sound, 
which quickly rises to a deep buzz accompanied by several notes, 
among which may be heard the note of the interrupting tuning 
fork ; as the temperature goes on increasing, these sounds die out 
again in the corresponding order, and when the whole wire is bright 
red, absolutely nothing can be heard. 
All these effects are explained by the magnetism of the steel. 
The first effect of heat is to destroy the 'permanent magnetism , which 
about 250° C. is practically insensible ; above this temperature the 
susceptibility for induced magnetism increases very fast, reaches a 
maximum about dull red, and then falls off again. 
Advantage was taken of Professor Tait’s thermoelectric diagram 
to verify the close connection between the magnetic, thermoelectric, 
and other characteristic physical properties of iron and its power of 
producing sounds, when traversed by a varying current of electri- 
city. The agreement was found to be very striking. 
Similar experiments were made with nickel, which is remarkable 
for the low temperature at which it loses its magnetic susceptibility. 
The behaviour of a nickel strip in the wire telephone was exactly in 
accordance with its magnetic properties. The results of thermo- 
metric and thermoelectric measurements, rendered the agreement still 
more remarkable. 
Cobalt, when magnetised and heated, gave first a minimum of 
sound and then an increase ; but no maximum was reached at the 
highest temperature (a bright red), to which I exposed it. This, 
again, is what is to be expected from its magnetic properties. 
Both with cobalt, and with steel which had been softened by 
heating to a high temperature, the effects due to permanent and to 
induced magnetism interfere, so that no period of absolute silence 
appears. Occasionally this interference produces very strong beats. 
A full account of the experiments above alluded to will be published 
in “Nature” (vol. xxii. No. 561, p. 303 — July 29, 1880). 
In the thermo-electric measurement above referred to I had the 
able assistance of Dr Knott, whose experience in such work is well 
known to the Society. The curves from which the above references 
were drawn were constructed by him, and will be given in the 
detailed account of the experiment to be published in “ Nature.” 
