AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
141 
magnetism is increased by stress, provided the magnetizing force does not reach a 
certain critical value, which however is very much less, other circumstances being the 
same, than is the case with nickel. It is possible, therefore, that if much smaller 
magnetizing forces had been employed in Experiment LXX., the increase of resistance 
caused by magnetism would be found to be heightened by the loads employed in that 
experiment. 
The Effects of Permanent Longitudinal Extension of Torsion and of 
Tempering on the Alteration of Electrical Resistance produced by 
Magnetizing. 
or 
The last experiment shows that moderate permanent logitudinal strain largely 
increases in the case of nickel the susceptibility to alteration of resistance from longi¬ 
tudinal magnetization. A similar effect is produced on iron, but as the whole point 
will be more fully discussed in Part III., it will suffice here to state that several 
experiments made according to the above plan, and also others where the comparison- 
wire and the wire to be tested were placed together in the magnetizing coil, proved, 
undoubtedly, that moderate permanent strain increases the susceptibility, but that 
this increase, after reaching a maximum, begins to decline, so that in some cases after 
the wire had been broken by the stress applied, the susceptibility appeared to be 
less than it was before the wire had been subjected to any strain. The above- 
mentioned maximum point depends upon the amount of magnetizing force in a 
manner to be hereafter described. 
The effect of permanent torsion up to a certain point was to diminish the suscepti¬ 
bility to alteration of resistance from longitudinal magnetization, and the amount of 
diminution was independent of the direction either of the M.C. or the B.C. 
The following experiment shows that in hard steel longitudinal magnetization 
increases the electrical resistance. 
Experiment LXII. 
A piece of a steel knitting-needle, 7'62 centims. in length and ’23 centim. in 
diameter, was hardened by heating it to a bright red and then plunging it into cold 
water. The piece was connected up with another of similar dimensions and similarly 
prepared, and both having been well covered with caoutchouc and silk, were made to 
form two branches of a “ Wheatstone’s bridge.” The coil B was used to impart 
magnetism, and with a large Leclanche for the B.C., and four Grove’s cells for 
the M.C., an increase of resistance represented by 30 divisions of the iridio-platinum 
wire was obtained. The brass clamps used to connect the pieces of steel with each 
other, and with the other branches of the “ bridge,” were so massive that even in this 
case their resistance is neglectable ; and since 30 divisions of the iridio-platinum wire 
would show an alteration of resistance of '010 per cent., we may assume that this last 
