444 
DR. J. HOPKmSON ON MAGNETIC AND OTHER PHYSICAL 
for different samples. It also appeared to be desirable to trace the resistance of iron 
wire up to and through the critical temperature, and to examine more particularly 
the phenomenon of recalescence, and determine the temperature at which it occurred. 
The most interesting results at which I have arrived may be shortly stated as 
follows :— 
For small magnetising' forces the magnetisation of iron steadily increases with rise 
of temperature till it approaches the critical temperature, when it increases very 
rapidly, till the permeability in some cases attains a value of about 11,000. The 
magnetisation then very suddenly almost entirely disappears. 
Tlie critical temperatures for various samples of iron and steel range from 690° C. 
to 870° C. 
Heating iron a little above the critical temperature does not entirely wipe out all 
effects of previous magnetisation. 
The temperature coefficient of electrical resistance is greater for iron than for other 
metals ; it increases greatly with increase of teinperature till the temperature reaches 
the critical temperature, when it suddenly changes to a value more nearly approaching 
to other metals. Kecalescence does occur at the critical temperature. The quantity 
of heat liberated in recalescence has been measured and is found to be quite com¬ 
parable with the heat required to melt bodies. 
Since making the experiments and writing the preliminary notes which have 
already appeared in the ‘ Proceedings of the Royal Society,’ my attention has been 
called to two papers which deal in part with some of the matters on which I liave 
been experimenting. Pionchon'" has shown that the specific heat of iron is very 
much greater at a red heat than at ordinary temperatures. W. Kohlrausch,! in an 
interesting paper, shows that, whereas the temperature coefficient of resistance of iron 
is much greater than usual for temperatures below the critical temperature, it 
suddenly diminishes on passing that temperature. He also identifies the temperature 
of recalescence with the critical temperature. So far as resistance of iron is con¬ 
cerned, W. Kohlrausch has anticipated my results, which I give, however, for the 
sake of completeness. 
Mcujnetic Expe ; -iments. 
The method of performing the magnetic experiments was the same as that used by 
Rowland. The copper wire was, however, insulated carefully with asbestos paper 
laid over the wire, and with layers of asbestos paper between the successive layers of 
the wire. The insulation resistance between the primary and the secondary coils was 
always tested, both at the ordinary temperature and at the maximum temperature 
used. At the ordinary temperature this resistance always exceeded a megohm; at 
* ‘ Comptes Rendus,’ vol. 103, p. 1122. 
t ‘ Wiedemann, Aunaleii,’ vol. 33, 1888. 
