The experiments of Kapitza (1929) showed that the increase of electrical resistance produced in a metal by a magnetic field
H
is not proportional to
H
2
, as was previously supposed. In the new experimental range made available by his method (Kapitza 1927) of producing very strong fields up to 300 kilogauss, Kapitza found that the increase of resistance tended towards a linear variation with the field strength. The result may be expressed in the formula
ΔR
/
R
0
=
b
(
H
-
H
k
), for
H
≫
H
k
, where
R
0
is the resistance at 0° C. This gives the asymptote to the experimental curve: but if experiments are made at field strengths up to a maximum
H
m
, and
H
m
≫
H
k
, then over a large part of the experimental range the curve obtained is practically identical with the asymptote. If the linear part of the curve is then extrapolated back to meet the axis of
H
, its intercept on that axis gives the parameter
H
k
, and the slope of the line gives the parameter
b
. If, however, the maximum field used is only of the order of
H
k
, the linear variation is only reached outside the experimental range; and some formula must be employed, in effect, to extrapolate to the region where the linear law holds, before the position of the asymptote and the values of the parameters can be derived. It is obvious that the values so obtained will vary according to the particular formula adopted.
The theory of the magneto-resistance effects in metals