142 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
number represents approximately the extent to which the conductivity of the steel 
was diminished by the longitudinal magnetization. 
Moreover, the steel under examination was well within the coi], the comparison- 
piece being, of course, outside, and also at right angles to the coil; and it was 
calculated that the increase per unit of resistance would for unit magnetizing force be 
•00010x 97 . , . _ . 
814x -68 x 4 7 r X 7= b9 ‘« * It) 8 , since c, the strength ot the magnetizing current here, 
approximately amounted to *2 absolute unit. 
It will be observed, by comparing this last result with the corresponding one in 
Experiment LXVIIL, that the alteration of resistance produced by a given magnetizing 
force is very much less with the hardened steel than that caused by the same magne¬ 
tizing force in a steel rod of the same diameter, but in the same condition as it was 
when received from the makers. Several other trials were made with the same piece 
of steel, in which smaller and smaller amounts of M.C. were employed, but in no case 
could any alteration of the nature of a decrease of resistance be observed. 
A similar experiment had been tried with pianoforte steel wire, hardened in the 
same way, and with various amounts of M.C., but the results were of the same nature, 
though much less in amount, as with the knitting-needle. It may be added that both' 
the knitting-needle and the wire were made so hard that they were quite brittle, and 
with both there was a permanent as well as a temporary increase of resistance 
produced by the magnetization. 
An attempt to determine Relations between the Alteration of Electrical 
Resistance produced by any Magnetizing Force, the Force itself, and 
the Magnetism induced by the Force. 
On consulting the fourth, fifth, and sixth columns of Table XXVI., it will be 
observed that the increase of resistance ensuing from magnetization depends not only 
upon the magnetism induced, but also upon the magnetizing force itself; and, in fact, 
we may say that if y denotes the increase of resistance, whilst a and /3 represent the 
magnetizing forces and the magnetism induced respectively, y=a.a + 6./3, where a, b 
are two constants. In the case of the nickel wire, a and /3 were measured in terms of 
the divisions of the scale of the tangent galvanometer, and of the scale of the Thomson’s 
reflecting galvanometer respectively ; whilst a and b were calculated from the observa¬ 
tions made with a=279 and a=S6. The agreement between the observed and 
calculated values of y is good, and certainly quite equal to that between the different 
observations made with the same values of a. Columns 4 and 5 show clearly that the 
alteration of resistance depends in this case more on the value of the magnetizing force 
than on the magnetism induced, and from the fact that - is nearly constant throughout, 
whereas ^ rapidly increases for the higher values of a, we are led to infer that the 
