-volts per degree C.

Platinum silver 6.62 micro-volts per degree C.

Nickelin 28.5 micro-volts per degree C.

German silver 10.43 micro-volts per degree C.

Manganin (St. Lindeck) 1.5 micro-volts per degree C.

Mechanically, the platinum silver is weak, and is greatly affected as
to its resistance by mechanical strains--in fact, Klemencic considers
it the worst substance he examined from this point of view--a
conclusion rather borne out by Mr. Glazebrook's experience with the
British Association standards already referred to (B. A. Reports, 1891
and 1892).

Taking everything into account, it will probably be well to construct
standards either with oil insulation only, or to bake the coils in
shellac before testing, instead of soaking in paraffin. Fig. 89
illustrates a form of an oil immersed standard which is in use in my
laboratory, and through which a considerable current may be passed.
The oil is stirred by means of a screw propeller.

Fig. 89.

Fig. 89 represents a standard resistance for making Clerk cell
comparisons by the silver voltameter method. The framework on which
the coils are wound consists of a base and top of slate. The pillars
are of flint glass tube surrounding brass bolts, and cemented to the
latter by raw shellac. Grooves are cut in the glass sleeves to hold
the wires well apart. These grooves were cut by means of a file
working with kerosene lubrication. A screw stirrer is provided, and
the whole apparatus is immersed in kerosene in the glass box of a
storage cell. The apparatus is aged to begin with by heating to a
temperature a good deal higher than any temperature it is expected to
reach in actual work. After this the rigidity of the frame is
intended to prevent any further straining of the wire. The apparatus
as figured is not intended to be cooled to 0 deg. C, so that it is put in
as large a box as possible to gain the advantage of having a large
volume of liquid. The top and bottom slates measure seven inches by
seven inches, and the distance between them is seven inches. The
inner coil is wound on first, and the loop which constitutes the end
of the winding is brought up to a suitable position for adjustment.
The insulation of the heavy copper connectors is by means of ebonite.

Sec. 120. Platinum Iridium.

Platinum 90 per cent, iridium 10 per cent. This material was prepared
in some quantity at the cost of the French