ed by Professors
Dewar and Fleming were, with some exceptions, not absolutely pure, but
in general represent the best that can be got by the most refined
process of practical metallurgy. We may note further that the
specific resistance is only correct for a temperature of about 15 deg. C,
since no correction for the expansion or contraction of material has
been applied.

The following notes on alloys suitable for resistance coils will
probably be found sufficient.

Sec. 117. Platinoid.

This substance, discovered by Martino and described by Bottomley
(Phil. Proc. Roy. Soc. 1885), is an alloy of nickel, zinc, copper, and
1 per cent to 2 per cent of tungsten, but I have not been able to
obtain an analysis of its exact composition. It appears to be
difficult to get the tungsten to alloy, and it has to be added to part
of the copper as phosphide of tungsten, in considerably greater
quantity than is finally required. The nickel is added to part of the
copper and the phosphide of tungsten, then the zinc, and then the rest
of the copper. The alloy requires to be remelted several times, and a
good deal of tungsten is lost by oxidation.

The alloy is of a fine white colour, and is very little affected by
air--in fact, it is to some extent untarnishable. The specific
resistance will be seen to be about one and a half times greater than
that of German silver, and the temperature coefficient is about 0.021
per cent per degree C. (i.e. about nineteen times less than copper,
and half that of German silver). To all intents and purposes it may
be regarded as German silver with 1 per cent to 2 per cent of
tungsten. It does not appear to have been particularly examined for
secular changes of resistance.

118. German Silver. This material has been exhaustively examined of
late years by Klemencic and by Feussner and St. Lindeck. Everybody
agrees that German silver, as ordinarily used for resistances, and
composed of copper four parts, zinc two parts, nickel one part, is
very ill-fitted for the purpose of making resistance standards. This
is due
(1) to its experiencing a considerable increase in resistance on
winding. Feussner and St. Lindeck found an increase of 1 per cent
when German silver was wound on a core of ten wire diameters.

(2) To the fact that the change goes on, though with gradually
decreasing rate, for months or years;

(3) to the fact that the resistance is permanently changed (increased)
by heati