608 
PROFESSOR J. A. EWING ON EXPERIMENTAL 
same factors were used to reduce to absolute measure the results of these two 
methods of experiment. The results of the two are, therefore, directly comparable 
with each other.* 
§ 91. Annealed Iron Wire .—In the group which I shall first describe of observations 
made in this way the wire of § 87 was used, but in the soft annealed state which 
preceded the stretching by 15^ kilos, there described. Curves of the relation of 
magnetisation to magnetising force were taken (May 1, 1882) with loads of 1,2, 4, 
and 6 kilos., as well as with no load, and after each curve taken with load, the load 
was removed and a curve without load taken, in order to see whether the loading 
had produced any permanent change in the quality of the metal. For brevity a 
curve of 3 and <*0, taken with no load on the wire, will be called a “ normal ” curve. 
The normal curves after each unloading agreed so very approximately with one another 
as to show that loading up to 6 kilos, did not sensibly harden the wire. It should be 
remembered that as the diameter was 0'77 mm. 1 kilo, of load corresponds to a stress 
of 2T5 kilos, per sq. mm. After each magnetisation the magnetising current was 
gradually reduced to zero, observations of 3 being made; the wire was then unloaded 
and demagnetised by reversals, a new load applied, and another curve of magneti¬ 
sation taken. 
The results are shown in Plate 64, fig. 44, where 3 and A are given in absolute 
units. The curves show the effects of gradually removing as well as gradually 
applying magnetising force, the effects of removal of ■§ being shown, however, only 
towards the end of that operation, in order not to confuse the figure. The general 
characteristics of the curves are these :—For equal low values of the magnetising 
force the magnetisation is greater in the loaded than the unloaded curves. But each 
loaded curve crosses the normal or unloaded curve at a value of 3 which is lower the 
higher the stress, and beyond that the magnetic susceptibility is less when the wire 
is under load than when it is not loaded. Finally, in the region of saturation, all the 
curves are converging, and it appears probable enough that with very great mag¬ 
netising forces the magnetisation would be the same for no load and for all values of 
the load. 
As regards the off curves, these preserve the relative places the on curves have in 
the region of saturation, that is to say, the normal curve is highest, and the curves 
for load lie lower the greater the load is. The distance between the off curves 
augments as the magnetising force approaches zero, and we then find that the loaded 
wire is much less retentive than the unloaded. 
cv 
The maximum value of k or A. is greater in the 1 kilo, curve than in any of the 
others. But the steepest slope or -A is to be found in the normal curve. 
* A few curves of magnetisation under load, similar to those about to be described, have recently been 
published by Mr. R. Shida, in a paper entitled “ Experimental Determinations of Magnetic Susceptibility 
and of Maximum Magnetisation in Absolute Measure,” Proc. Roy. Soc., No. 227, 1883. 
