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of zinc as compared with brass, proposed to use a steel rod with a
collar at the bottom, on which rested a hard drawn zinc rod. From this
rod hung a steel tube to which the bob was attached. The total length
of the steel rod and of the steel tube down to the centre of the bob
was made to the total length of the zinc tube, in the ratio of 5 to 2
(being the ratio of the expansions of zinc and steel); for a 39.14 in.
pendulum we should therefore want a zinc tube equal in length to 2/3
(39.14) = 26-1/4 in. In practice the zinc tube is made about 27 in. long,
and then gradually cut down by trial. In fact the weight of a heavy
pendulum squeezes the zinc, and it is impossible by mere theory to
determine what will be its behaviour. The zinc tube must be of rolled
zinc, hard drawn through a die, and must not be cast. Ventilating
holes must be made in suitable places in the steel tube and the collar
on which it rests, to ensure that changes of temperature are rapidly
communicated throughout the system.

A pendulum with a rod of dry varnished deal is tolerably compensated
by a bob of lead or of zinc 10-1/2 to 13 in. in height, resting on a nut
at the bottom of the rod.

Invar.

The old methods of pendulum compensation for heat may now be
considered as superseded by the invention of "invar," a combination of
nickel and steel, due to Charles E. Guillaume, of the International
Office of Weights and Measures at Sevres near Paris. This alloy has a
linear coefficient of expansion on the average of .000001 per degree
centigrade, that is to say, only about 1/11 that of ordinary steel.
Hence it can be easily compensated by means of brass, lead or any
other suitable metal. Brass is usually employed. In the invar pendulum
introduced into Great Britain by Mr Agar Baugh a departure is made
from the previous practice of merely calculating the length of the
compensator, fastening it to the lower part of the pendulum, and
attaching it to the centre of the bob. In the case of these pendulums,
accurate computations are made of the moments of inertia of every
separate individual part. Thus, for instance, since an addition of
volume due to the effect of heat to the upper part of the bob has a
different effect upon the moment of inertia from that of an equal
quantity added to the lower part of the bob, the bob is suspended not
from its centre, but from a point about 1/