ELECTRODYNAMIC QUALITIES OP METALS. 
79 
was no doubt due to the magnetizing influence of the earth tending to reverse the 
retained magnetism of the bar. It will be further investigated in a continuation of 
experiments on cobalt. 
235. The effect of longitudinal stress on the magnetization of nickel when magnetized 
by a current flowing in a coil surrounding the bar formed the subject of the next 
series of investigations. The magnetizing coil was 54 centims. long, and consisted of 
six layers of silk-covered copper wire of No. 22 B.W.G., each layer forming a solenoid 
containing 10'7 turns per centim. The resistance of the coil when cool was 7'2 ohms, 
and the resistance of the electrodes '3 of an ohm. In this experiment the magneto¬ 
meter was placed at a distance of 40 centims. from the axis of the bar, and on a level 
with its lower end ; and in order that the deflection due to the total magnetization of 
the bar might be conveniently measured, the directive force on the magnetometer 
needle was increased by placing behind it, in the magnetic meridian, a bar magnet, 
with its true north pole turned towards the north. As in the previous experiments 
with iron and steel, — M indicates that the electromagnetic field was opposite in 
polarity to that of the earth, and +M that its polarity was the same as that of the 
earth. The dead-beat galvanometer used in the experiments on the effects of trans¬ 
verse stress on the magnetization of an iron tube was again employed to measure 
the strength of the magnetizing current. 
236. The results of a series of these experiments are given in Table IV. It will 
be seen from that table that the effects of the application and removal of stress were 
respectively to diminish and to increase the induced magnetization, and that, as in 
the case of soft iron, this effect reached a maximum with a certain strength of mao- 
netizing current, after which it slowly diminished. In this experiment the critical 
value of the magnetizing force corresponding to what has been called above, in the 
account of experiments in soft iron, the Villari critical value, was not reached. 
Table IV.—Bar of Wrought Nickel. 
1 
Operations. 
Readings. 
Zero 554. 
Differences. 
Total 
magnetization. 
Strength of 
magnetizing 
current. 
Bar put in position . 
352 
-202 
+ 202 
-M . . . 
583 
+ 231 
- 29 
62 
10 “ ons” and “ offs ” 
(2 double cells) 
with 285 lbs. . . 
637 
+ 54 
- 83 
285 lbs. on ... 
623 
- 14 
- 69 
„ off ... . 
637 
+ 14 
83 
B . . . . 
568 
- 69 
- 14 
285 lbs. on . 
554 
- 14 
0 
„ off ... . 
556 
+ 2 
- 2 
„ on ... . 
553 
- 3 
+ 1 
„ off ... . 
555 
+ 2 
- 1 
