gether with the lost motion which is likely to exist in the mechanism
connected to the pointer, makes the expansion pyrometer unreliable; it
should be used only when its limitations are thoroughly understood and
it should be carefully calibrated. Unless the brass and iron are known
to be of the same temperature, its action will be anomalous: for
instance, if it be allowed to cool after being exposed to a high
temperature, the needle will rise before it begins to fall. Similarly, a
rise in temperature is first shown by the instrument as a fall. The
explanation is that the iron, being on the outside, heats or cools more
quickly than the brass.

Calorimetry--This method derives its name from the fact that the process
is the same as the determination of the specific heat of a substance by
the water calorimeter, except that in one case the temperature is known
and the specific heat is required, while in the other the specific heat
is known and the temperature is required. The temperature is found as
follows:

A given weight of some substance such as iron, nickel or fire brick, is
heated to the unknown temperature and then plunged into water and the
rise in temperature noted.

If X = temperature to be measured, w = weight of heated body in pounds,
W = weight of water in pounds, T = final temperature of water, t =
difference between initial and final temperatures of water, s = known
specific heat of body. Then X = T + Wt / ws

Any temperatures secured by this method are affected by so many sources
of error that the results are very approximate.

Thermo-electric Pyrometers--When wires of two different metals are
joined at one end and heated, an electromotive force will be set up
between the free ends of the wires. Its amount will depend upon the
composition of the wires and the difference in temperature between the
two. If a delicate galvanometer of high resistance be connected to the
"thermal couple", as it is called, the deflection of the needle, after a
careful calibration, will indicate the temperature very accurately.

In the thermo-electric pyrometer of Le Chatelier, the wires used are
platinum and a 10 per cent alloy of platinum and rhodium, enclosed in
porcelain tubes to protect them from the oxidizing influence of the
furnace gases. The couple with its protecting tubes is called an
"element". The elements are made in different lengths to suit
conditions.

It is not necessary for accuracy to expose the whole l