The thermometer is filled by pouring in mercury through a small funnel
until the level of the mercury (when the thermometer is at the desired
temperature) is about 1-1/2 inch below the T. The piece of glass tube A,
closed at its upper extremity by the cork I, through which the fine glass
tube B passes into the stem of the thermometer, is now filled by means of
the perforated cork at its lower extremity on the stem of the thermometer.
The gas supply tube is attached to the top of the tube A, the burner to
the T, so that the gas passes in at the top, down the fine tube B, rises
in the space between B and the inside wall of the stem of the thermometer,
and escapes by the T. The regulator is set for any given temperature by
pushing the cork C, and consequently the tubes A and B, which are firmly
attached to it, up or down the stem of the thermometer, until the
regulator just cuts off the gas at the desired temperature.

[Illustration: FIG. 49.--PAGE'S REGULATOR.]

[Illustration: FIG. 50.--PAGE'S GAS REGULATOR, SHOWING BYE-PASS AND
CUT-OFF ARRANGEMENT.]

As soon as the temperature falls, the mercury contracts, and thus opens
the end of the tube B. The gas is thus turned on, and the temperature
rises until the regulator again cuts off the gas. In order to prevent the
possible extinction of the flame by the regulator, the brass tube which
carries the gas to the regulator is connected with the tube which brings
the gas from the regulator to the burner by a small brass tap (Fig. 2).
This tap forms an adjustable bye-pass, and thus a small flame can be kept
burning, even though the regulator be completely shut off. It is obvious
that the quantity of gas supplied through the bye-pass must always be less
than that required to maintain the desired temperature. This regulator,