bsolute zero of temperature
upwards, then it is found that these temperature-resistance lines are
curved lines having their convexity either upwards or downwards. In
other words, the second differential coefficient of resistance with
respect to temperature is either a positive or negative quantity. An
extensive series of observations concerning the form of the
resistivity curves for various pure metals over a range of temperature
extending from -200 deg. C. to +200 deg. C. was carried out in 1892
and 1893 by Fleming and Dewar (_Phil. Mag._ Oct. 1892 and Sept. 1893).
The resistance observations were taken with resistance coils
constructed with wires of various metals obtained in a state of great
chemical purity. The lengths and mean diameters of the wires were
carefully measured, and their resistance was then taken at certain
known temperatures obtained by immersing the coils in boiling aniline,
boiling water, melting ice, melting carbonic acid in ether, and
boiling liquid oxygen, the temperatures thus given being +184 deg..5
C., +100 deg. C., 0 deg. C., -78 deg..2 C. and -182 deg..5 C. The
resistivities of the various metals were then calculated and set out
in terms of the temperature. From these data a chart was prepared
showing the temperature-resistance curves of these metals throughout a
range of 400 degrees. The exact form of these curves through the
region of temperature lying between -200 deg. C. and -273 deg. C. is
not yet known. As shown on the chart, the curves evidently do not
converge to precisely the same point. It is, however, much less
probable that the resistance of any metal should vanish at a
temperature above the absolute zero than at the absolute zero itself,
and the precise path of these curves at their lower ends cannot be
delineated until means are found for fixing independently the
temperature of some regions in which the resistance of metallic wires
can be measured. Sir J. Dewar subsequently showed that for certain
pure metals it is clear that the resistance would not vanish at the
absolute zero but would be reduced to a finite but small value (see
"Electric Resistance Thermometry at the Temperature of Boiling
Hydrogen," _Proc. Roy. Soc._ 1904, 73, p. 244).

The resistivity curves of the magnetic metals are also remarkable for
the change of curvature they exhibit at the magnetic critical
temperature. Thus J. Hopkinson and D.