The result is that
no loss of heat is possible in the instrument, and the water in the
central tube merely takes up just so much heat as is conducted into it
through the metal of the explorer. This heat it brings back through a
short India-rubber pipe to a casing containing a thermometer. This
thermometer is immersed in the returning current of water, and records its
temperature. It is graduated by immersing the instrument in known and
constant temperatures, and thus the graduations on the thermometer give at
once the temperature, not of the current of water, but of the medium from
which it has received its heat. In order to render the instrument
perfectly reliable, all that is necessary is that the current of water
should be always perfectly uniform, and this is easily attained by fixing
the size of the outlet once for all, and also the level of water in the
tank. So arranged, the pyrometer works with great regularity, indicating
the least variations of temperature, requiring no sort of attention, and
never suffering injury under the most intense heat; in fact the tube, when
withdrawn from the furnace, is found to be merely warm. If there is any
risk of the instrument getting broken from fall of materials or other
causes, it may be fitted with an ingenious self-acting apparatus shutting
off the supply.
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