DR, J. HOPKINSON ON THE MAGNETISATION OF IRON. 
461 
Sample. 
Area from curve. 
4 coercive force x max. induction 
47r 
4i r 
No. I. 
17247 
13356 
„ HI. 
15139 
13037 
„ VI. 
45903 
40120 
„ VII. 
61898 
65786 
„ VIII. 
50521 
- 42366 
„ IX. 
74371 
99401 
In this we note that for soft iron the area is greater than the product, the reverse 
for hard steel; for any practical purpose we may assume that the greatest dissipation 
of energy which can be caused by a complete reversal to and fro of magnetisation is 
, , , . coercive force x maximum induction . . , . „ 
approximately measured by ---. An interesting feature 
in the curves is the manner in which the residual magnetism rapidly attains to near 
its maximum value, and is then nearly constant, whilst the induction continues to 
increase. This is very marked in the case of cast iron. 
The column of figures in the general table of results almost explain themselves. 
In the case of the cast iron, the total and the graphitic carbon are given, the 
difference being 1 the combined carbon. In the case of the manganese steel and iron, 
the induction is almost proportional to the magnetising force, hence permeability is 
really the magnetic property to be noted : this is given below in a separate table. 
The demagnetising force is that reverse force which, when applied after great magnet¬ 
ising force, just suffices to remove all permanent magnetisation. The energy dissi- 
, , . OA x maximum induction , . , , . , 
pated is -, and is approximately tne energy m ergs, converted 
into heat in a complete cycle of magnetisation from the limit in one direction to that 
in the opposite and back again. 
In the general table of results one of the striking features is the high specific 
resistance of some samples of cast iron, ten times as great as wrought iron. This 
fact is not without practical importance in some forms of dynamo machines, for the 
energy wasted by local currents induced in the iron by given variations of the 
magnetic force will be but roth as great with cast iron as with wrought iron. The 
high resistance of cast iron may be due in large measure to its heterogeneity ; grey 
cast iron may be regarded as a mechanical mixture of more or less pure iron with 
very small bits of graphite. 
[Jan. 15, 1886.—I have recently determined the rate of variation with temperature 
of the electric resistance of a sample of cast iron for the purpose of ascertaining 
whether it approximated more nearly to a pure metal, to an alloy, or to bodies the 
resistance of which decreases with rise of temperature. The sample examined was 
a thin rod of grey iron 6'71 millims. diameter and 24'85 centims. long between the 
contacts. The range of temperature was 10° C. to 130° C., and through this range 
MDCCCLXXXV. 3 O 
