om a
mercury column, although the thermometer bulb is filled with alcohol. It
is an advantage to make the dumb-bell-shaped rods of iron, as the
thermometer can then be reset by the use of a small magnet, another
advantage consequent on the use of metal being that the rods can be
easily adjusted, by slight bending, so as to remain stationary in the
tubes when the thermometer is hanging vertically, and yet to move with
sufficient freedom to yield to the pressure of the recording column.

The thermometer may be filled by the following method:--When the
straight tube has been made the first dumb-bell is introduced and shaken
down well towards the lower bulb, the tube is now bent to its final
shape and the whole thermometer filled with alcohol as described on page
44. Now heat the thermometer to a little above the maximum temperature
that it is intended to record, and pour clean mercury into the open bulb
while holding the thermometer vertically. Allow to cool, and the mercury
will be sucked down. The second dumb-bell is now introduced, sufficient
alcohol being allowed to remain in the open bulb to about half fill it,
and the alcohol in this bulb is boiled to expel air. The tube through
which the bulb was filled in now sealed.

_Clinical Thermometers._--The clinical thermometer is a maximum
thermometer of a different type. In this case there is a constriction
of the bore at a point just above the bulb. When the mercury in the bulb
commences to contract, the mercury column breaks at the constriction and
remains stationary in the tube, thus showing the maximum temperature to
which it has risen.

_Vacuum Tubes._--There are so many forms of these that it is scarcely
practicable or desirable to give detailed instructions for making them;
but an application of the various methods of glass-working which have
already been explained should enable the student to construct most of
the simpler varieties. An interesting vacuum tube is made which has no
electrodes, but contains a quantity of mercury. When the tube is rocked
so as to cause friction between the mercury and the glass sufficient
charge is produced to cause the tube to glow.

_A Sprengel Pump._--This, in its simplest form, is illustrated by _a_,
Fig. 12. Such a form, although highly satisfactory in action, needs
constant watching while in action, as should the mercury funnel become
empty air will enter the exhausted vessel. Obviously, the fall-tube must
be made not less th