IMPROVED ELECTRIC FIRE ALARM.


The object of this apparatus is to close an electric circuit when the
temperature of a room rises above a certain point. Many devices have
been invented for effecting this object, each of which have their own
advantages or disadvantages. The invention of Mr. Pritchett enables
the required result to be obtained in a very satisfactory manner. 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