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Page 27
In order to prepare the apparatus for carriage, the winch is taken off
and placed in the compartment, _m_, which is closed by means of a
door, Q.
Figs. 5 and 6 show the arrangement of the dynamite cartridges and
wires in the blast hole. Figs. 7 to 10 show different arrangements of
the igniting wires. Figs. 11 and 12 give the general arrangement for
igniting a number of cartridges simultaneously by means of the
electric machine. Fig. 13 shows the arrangement where powder is
employed. Fig. 14 shows the arrangement of a horizontal
hole.--_Annales Industrielles._
* * * * *
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
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.
Owing to the surface which the glass vessel exposes, the air inside
quickly responds to any external change of temperature, consequently
the apparatus is very sensitive. Another important feature is the fact
that the cradle and vessel in canting over acquires a certain
momentum, and thus the contact made becomes very certain.
[Illustration: PRITCHETT'S ELECTRIC FIRE ALARM.]
Mr. Pritchett proposes that his apparatus shall give external evidence
outside the house by ringing a gong, and by dropping a semaphore arm
released by an electromagnet. He also proposes (as has often been
suggested) that a water supply shall be automatically turned
on.--_Electrical Review._
* * * * *
A STANDARD THERMOPILE.
Dr. G. Gore, F.R.S., has invented an improved thermopile for
measuring small electromotive forces. It consists of about 300 pairs
of horizontal, slender, parallel wires of iron and German silver, the
former being covered with cotton. They are mounted on a wooden frame.
About 1� in. of the opposite ends of the wires are bent downward to a
vertical position to enable them to dip into liquids at different
temperatures contained in long narrow troughs; the liquids being
non-conductors, such as melted paraffin for the hot junctions, and the
non-volatile petroleum, known as thin machinery oil. The electromotive
force obtained varies with the temperature; a pile of 295 pairs having
a resistance of 95.6 ohms at 16 deg. Cent. gave with a difference of
temperature of 100 deg. Cent. an electromotive force of 0.7729 volts,
or with 130 deg. Cent. an electromotive force of 1.005 volt. Each
element, therefore, equaled 0.0000262 volt for each degree Cent.
difference of temperature. On having been verified with a standard
voltaic cell the apparatus becomes itself a standard, especially for
small electromotive forces. It is capable of measuring the 1/34861
part of a volt. For higher electromotive forces than a volt, several
of these piles would have to be connected in series. The fractional
electromotive force is obtained by means of a sliding contact which
cuts out so many pairs as is required.
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