ng to 40 deg. C. or over. By "artificially ageing" coils of
German silver by heating to 150 deg. C, say for five or six hours, its
permanency is greatly improved, and it becomes fit for ordinary
resistance coils where changes of, say, 1/5000 do not matter.

It is a remarkable property of all nickel alloys containing zinc that
their specific resistance is permanently increased by heating, whereas
alloys which do not contain zinc suffer a change in the opposite
direction. The manufacturers of German silver appear to take very
little care as to the uniformity of the product put on the market;
some so-called German silver is distinctly yellow, while other samples
are bright and white.

It is noted by Price (Measurements of Electrical Resistance, p. 24)
that German silver wire is apt to exhibit great differences of
resistance within quite short lengths. This has been my own
experience as well, and is a great drawback to the use of German
silver in the laboratory, for it makes it useless to measure off
definite lengths of wire with a view to obtaining an approximate
resistance. In England German silver coils are generally soaked in
melted hard paraffin. In Germany, at all events at the Charlottenburg
Institute, according to St. Lindeck--coils are shellac-varnished and
baked. In any case it appears to be essential to thoroughly protect
the metal against atmospheric influence.

Sec. 119. Platinum Silver.

In the opinion of Matthieson and of Klemencic the 10 per cent silver,
90 per cent platinum alloy is the one most suitable for resistance
standards. At all events, it has stood the test of time, for, with
the following exceptions, all the British Association coils
constructed of it from 1867 to the present day have continued to agree
well together. The exceptions were three one-ohm coils, which
permanently increased between 1888 and 1890, probably through some
straining when immersed in ice. One coil changed by 0.0006 in 1
between the years 1867 and 1891. According to Klemencic, absolute
permanency is not to be expected even from this alloy.

Its recommendation as a standard depends on its chemical inertness,
its small temperature coefficient (0.00027 per degree), and its small
thermo-voltage against copper, as the following table (taken from
Klemencic) will show:-

Thermo-voltages in Micro-volts per degree against Copper
over the Range 0 deg. to 17 deg. C.

Platinum iridium 7.14 micro