From
100 deg. to 220 deg.F. ether is the liquid used, from thence to 680 deg. it is water,
and above the latter temperature mercury is employed.
Another class of pyrometers having great promise in the future is based on
what may be called the "water-current" principle. Here the temperature is
determined by noting the amount of heat communicated to a known current of
water circulating in the medium to be observed. The idea, which was due to
M. De Saintignon, has been carried out in its most improved form by M.
Boulier. Here the pyrometer itself consists of a set of tubes one inside
the other, and all inclosed for safety in a large tube of fireclay. The
central tube or pipe brings in the water from a tank above, where it is
maintained at a constant level. The water descends to the bottom of the
instrument, and opens into the end of another small tube called the
explorer (_explorateur_). This tube projects from the fireclay casing into
the medium to be examined, and can be pushed in or out as required. After
circulating through this tube the water rises again in the annular space
between the central pipe and the second pipe. The similar space between
the second pipe and the third pipe is always filled by another and much
larger current of water, which keeps the interior cool.
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