The
apparatus consists (as shown by the figure) of a long glass vessel
containing air; connected to this vessel there is a glass tube filled
with mercury. The whole is mounted on a metal cradle, which turns on
pivots. According to the position which the glass vessel and its
adjuncts occupy in the cradle (this position being adjustable by means
of a thumb-screw, seen at the upper part of the cradle), so will the
same have a tendency to rock longitudinally over to one side or the
other. Now, if we suppose the position to be such that the right hand
end of the glass vessel is depressed, and the left hand end raised,
then if the vessel becomes subjected to an elevation of temperature,
the air inside the same will become expanded, and the mercury column
in the tube will be driven over to the left, and will rise in the
turned up end of the tube. This will cause the left hand branch of the
glass vessel, and its attachments, to become increased in weight,
while the right hand branch will become proportionally lighter; the
consequence of this will be that the vessel and its cradle will cant
over, and by falling on an electrical contact will close a circuit and
sound an alarm. It is obvious that the apparatus is equally well
adapted for indicating a diminution as well as an increase of
temperature, for if the electrical contact be placed under the right
hand portion of the cradle, and the latter be adjusted so that in its
normal position its left hand portion is depressed, then when the
glass vessel becomes cooled, the air in it will contract, and the
mercury will fall in the turned-up portion of the tube before referred
to, and will rise in the limb connected to the vessel, consequently
the cradle and glass vessel will cant over in the reverse way to that
which it did in the first case.
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