84 
SIR WILLIAM THOMSON ON THE 
§§241-244. Experiment by the direct Magnetometric Method on the effects of 
longitudinal stress on the Magnetization of Iron Wire. 
241. In this experiment the magnetizing coil described above (§ 201) was employed. 
The total length of the wire (which was a piece of Messrs. Johnson and Nephew’s very 
soft iron wire, and cut from the same hank as that used in the former experiments) was 
97 centims. As only the soldered fastenings of the wire projected beyond the ends of 
the coil, the magnetometer needle was placed on a level with the top of the coil, and at 
a distance from its axis of 25 centims., and the distance of the scale of the magnetometer 
from the mirror was 108 centims. 
242. This experiment confirmed in all essential points the results obtained by the 
ballistic method in the experiments described above (§§ 200-212). The effects of 
applying and removing a weight of 14 lbs. were respectively to increase and to diminish 
by 31 divisions the magnetism induced in the wire by the vertical component of the 
earth’s magnetic force. The amount of this induced magnetism, when only the pan 
was hanging on the wire, was also 31 divisions. Hence the application and removal 
of the 14 lbs. alternately doubled, and reduced to its previous amount, the magneti¬ 
zation induced by the earth’s force. 
When the magnetizing current was 4‘25 divisions of the battery-galvanometer 
scale, the application and removal of pull produced no effect, thus showing that the 
influence of the earth’s magnetizing force was exactly counterbalanced by that due 
to the current. The maximum effect of applying and removing stress was obtained 
when the magnetizing force was about 50 divisions (one cell gave 70 divisions). The 
Villaei critical value was obtained with 215 divisions of magnetizing current, or 
50 times the magnetizing force which balanced the influence of the earth’s magnetic 
force’. This is a much greater number than that obtained by the ballistic method ; that 
it is so is no doubt due to the fact that the induction-coil then used was much shorter 
than the magnetizing coil, and was placed with the centre of its length coincident with 
that of the magnetizing coil. With the highest strength of current (567 divisions) 
the effect of “ on” and “ off” was 8‘5 divisions, and the effects were, as in the former 
experiments, increasing very slowly. 
243. The value in absolute units of the vertical component of the earth’s magnetic 
force was calculated from the value of the magnetizing current which balanced it, and 
the total resistance of coil electrodes and battery, which were all measured for the 
purpose with great exactness. As before, the electromotive force of one tray cell was 
taken as one volt, or 10 s on the C. G. S. system of units. The total resistance with 
one cell in circuit was 3'503 ohms, the number of turns per centimetre in the coil 
19 - 628, and the fraction of the cell which gave a magnetizing force which just 
balanced the earth ‘0607 of that due to one cell. Hence we have vertical component 
4tt x -0607 x 10 8 x 19-628 1 . 1 , , . x x x ru 
=-wxx —vxq -= 429, which must be very nearly its true value at Glasgow. 
3-503 x 10 9 j j s 
(Compare with §207 above.) 
