ELEOTRODYNAMIC QUALITIES OF METALS. 
83 
to approach. It was found more convenient to continue the experiment with a smaller 
bar of nickel, kindly lent for the purpose by Professor Tait. This (which was a square 
bar 457 centims. long and ‘30 centirn. thick) was placed within a coil wound on 
a thin copper tube of just sufficient internal diameter to admit the bar. The coil 
contained six layers of silk-covered copper wire, of No. 22 B.W.G., each layer forming 
a solenoid 427 centims. long, containing 107 turns per centirn. The total resistance 
of the coil when cool was 4*33 ohms. The resistance of the electrodes was, as before, 
•3 of an ohm. The magnetometer needle was in this case placed on a level with the 
upper end of the bar, and at a distance of 25 centims. from the axis of the bar, and of 
108 centims. from the scale of half millims. on which the readings were observed. 
The stress was not applied by means of the lever, but a weight of 14 lbs. was placed on 
a pan attached to the bar. This pan, which weighed 1 lb., was left hanging on the bar 
during the whole experiment. 
239. In their general character the results are precisely similar to those shown in 
Table IV. A maximum effect of 20 divisions was obtained when the magnetizing force 
was 194 divisions of the battery galvanometer scale, or that due to about 4 cells. As 
the magnetizing force was increased beyond this value the effects obtained gradually 
diminished, and seemed to reach zero when the magnetizing force was about 1000 
divisions, or that due to about 40 cells. The results at this point could not, on 
account of variations of the magnetizing force due to heating of the coil, be relied on 
as being accurate. 
[Note added June 4, 1879.-—This result has not been confirmed by experiments 
lately made with improved apparatus in which the effects of the heating of the 
magnetizing coil, formerly a great source of trouble, were to a great extent prevented. 
No sign of a neutral point was found, although battery powers of from 5 to 79 tray 
cells were employed for the purpose of magnetizing the nickel bar, which was the 
actual bar formerly experimented on.] 
240. An experiment was then made to find whether this critical value could, as in 
the case of the soft iron tube, he obtained with a smaller degree of magnetizing force 
when the magnetometer was placed on a level with a point between the middle 
and end of the bar. Accordingly, the magnetometer was lowered 10 centims., and 
brought to a distance of 10 centims. from the axis of the bar. The scale was left 
in its former position, and thus the distance of the mirror from it was increased to 123 
centims. The directive force on the needle was also increased to 6'08 times that due 
to the horizontal component of the earth’s magnetic force. The maximum effect of 
the application and removal of stress was obtained with a magnetizing force of 50 
scale-divisions, or that due to 1 cell. And the critical value was found without 
difficulty to be 428 divisions of the battery galvanometer scale, or the current due to 
10 cells. Beyond that point the effect of “ on ” and £; off” were respectively to increase 
and to diminish the induced magnetization of the bar, the effect with 707 divisions, or 
20 cells, being about 3| divisions of the scale. 
m 2 
