ld not cause
it to boil. The rise in temperature of the water was measured by a
thermometer, and, knowing the heat capacity of the iron ball and
that of the water, the temperature of the ball, and therefore the
furnace, could be calculated. Usually a set of tables was prepared
to simplify the calculations. The iron ball, however, scaled, and
changed in weight with repeated use, making the determinations
less and less accurate. A copper ball was often used to decrease
this change, but even that was subject to error. This method is
still sometimes used, but for uniform results, a platinum ball,
which will not scale or change in weight, is necessary, and the
cost of this ball, together with the slowness of the method, have
rendered the practice obsolete, especially in view of modern
developments in accurate pyrometry.

PYROMETERS

Armor plate makers sometimes use the copper ball or Siemens' water
pyrometer because they can place a number of the balls or weights on
the plate in locations where it is difficult to use other pyrometers.
One of these pyrometers is shown in section in Fig. 109.

SIEMENS' WATER PYROMETER.--It consists of a cylindrical copper vessel
provided with a handle and containing a second smaller copper vessel
with double walls. An air space _a_ separates the two vessels, and
a layer of felt the two walls of the inner one, in order to retard
the exchange of temperature with the surroundings. The capacity
of the inner vessel is a little more than one pint. A mercury
thermometer _b_ is fixed close to the wall of the inner vessel,
its lower part being protected by a perforated brass tube, whilst
the upper projects above the vessel and is divided as usual on the
stem into degrees, Fahrenheit or Centigrade, as desired. At the
side of the thermometer there is a small brass scale _c_, which
slides up and down, and on which the high temperatures are marked
in the same degrees as those in which the mercury thermometer is
divided; on a level with the zero division of the brass scale a
small pointer is fixed, which traverses the scale of the thermometer.

[Illustration: FIG. 109.--Siemens' copper-ball pyrometer.]

Short cylinders _d_, of either copper, iron or platinum, are supplied
with the pyrometer, which are so adjusted that their heat capacity at
ordinary temperature is equal to one-fiftieth of that of the copper
vessel filled with one pint of water. As, however, the specific heat
of metals increases with