Government, and distributed
for test about 1886. Klemencic got some of it as representing
Austria, and found it behaved very like the platinum silver alloy just
discussed. The temperature coefficient is, however, higher than for
platinum silver (0.00126 as against 0.00027). The mechanical
properties of the alloy are, however, much better than those of the
silver alloy; and in view of the experience with B. A. standards
above quoted, it remains an open question whether, on the whole, it
would not be the better material for standards, in spite of its
higher price. Improvements in absolute measurements of resistance,
however, may render primary standards superfluous.

Sec. 121. Manganin.

Discovered by Weston--at all events as to its application to
resistance coils. A glance at the diagram will exhibit its unique
properties, on account of which it has been adopted by the
Physikalisch Technischen Reichsanstalt for resistance standards. The
composition of the alloy is copper 84 per cent, manganese 12 per cent,
nickel 4 per cent, and it is described as of a steel-gray colour.
Unfortunately it is apt to oxidise in the air, or rather the manganese
it contains does so, so that it wants a very perfect protection
against the atmosphere.

Like German silver, manganin changes in resistance on winding, and
coils made of it require to be artificially aged by heating to 150 deg.
for five hours before final adjustment. The annealing cannot be
carried out in air, owing to the tendency to oxidation. The method
adopted by St. Lindeck (at all events up to 1892) is to treat the
coil with thick alcoholic shellac varnish till the insulation is
thoroughly saturated, and then to bake the coil as described. The
baking not only anneals the wire, but reduces the shellac to a hard
and highly insulating mass.

Whether stresses of sufficient magnitude to produce serious mechanical
effects can be set up by unequal expansion of wire and shellac during
heating and cooling is not yet known, but so far as tested (and it
must be presumed that the Reichsanstalt tests are thorough) no
difficulty seems to have been met with. In course of time, however,
probably the best shellac coating will crack, and then adieu to the
permanency of the coil! This might, of course, be obviated by keeping
the coil in kerosene, which has no action on shellac, but which
decomposes somewhat itself.

The method of treatment above described suffices to render c