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Construction of a Thermometer.--The first thing to be attended to in this connection,
is the selection of the tube to be used for the stem of the instrument.
As to the bore of the tube, it can easily be seen that the smaller
this is, the greater will be the sensitiveness of the thermometer,
assuming the bulb to have the same size in all cases; or, on
the other hand, for a given sensitiveness, the size of the bulb
can be diminished, according as a tube of a finer bore is employed.
The exact relation between the bore and the size of the bulb
may be determined as follows :-
-
- [calculations for examples with spherical
and cylindrical bulbs]
- . . .
-
- Thermometer Tubes should be of Uniform
Bore.--In selecting a tube for the
construction of a thermometer, it is most important to determine
whether the bore is uniform throughout the
 length
to be used. This point may be settled by carefully sucking a
short thread of mercury into the tube, and measuring its length
when occupying various positions in it. A tube in which these
lengths vary by more than a very small amount should be discarded.
- The bulb of the thermometer is sometimes
blown directly from the glass composing the tube, but more often
is made independently and fused on to the stem. Before doing
so, the inside of the tube is carefully cleaned, as any traces
of dust or other foreign matter will subsequently cause great
trouble and annoyance.
- Filling the Thermometer.--Before the bulb has been sealed on to one end of
the stem, a thistle funnel A is blown on the other end of the
latter. The tube is also drawn out at the point B where the thermometer
is to be sealed off. In doing this, care must be taken to pull
the tube out as little as possible, but to allow the glass to
collapse so as to leave only a very fine aperture, the walls
remaining thick. It is further worth while to blow a small expansion,
C, at a point just above the position selected for the graduation
marking the highest temperature which the thermometer is required
to measure. By this means accidental breakage of the thermometer
through a small overheating is guarded against. The funnel A
having been filled with pure dry mercury, the bulb D is slightly
heated so as to drive out some of the Imprisoned air. On allowing
D to cool, mercury will be drawn in. Amateurs often heat the
bulb too much to start with, resulting in a breakage due to the
cold mercury suddenly cooling the hot glass. When once a small
amount of mercury has been drawn into the bulb there is less
fear of this mishap, since the bulb is then not likely to be
heated to a greater temperature than that of boiling mercury.
Subsequent heatings and coolings will suffice to entirely fill
the bulb and stem with mercury. Finally the whole of the contained
mercury must be boiled. This cannot he done without considerable
risk when nothing further than a naked flame is used. Greater
safety is attained by placing the thermometer, together with
its attached thistle funnel filled with mercury, in an enclosure
which can be heated to a sufficiently high temperature, and subsequently
allowed to cool gradually. The mercury is boiled in order to
drive off the air which otherwise always clings to the walls
of the tube.
- In order to seal the thermometer off, the
mercury in the bulb and stem is raised to a temperature sufficiently
above that which is to correspond to the highest graduation;
this temperature will be that at which the thermometer will burst
after being sealed off. A small pointed flame is then directed
on to the constriction B, Fig. 4, and the mercury having been
evaporated from the neighborhood of the point of the flame, the
temperature is increased till the tube fuses and the walls fall
together, when the upper part may be pulled off.
- Determination of the Fixed Points.--The determination of the fixed points of a thermometer
should be postponed for at least a week after the thermometer
has been filled and sealed. The most convenient fixed points
for a thermometric scale are those corresponding to the melting
of pure ice in distilled water, and the boiling point of water
at standard atmospheric pressure.
- In order to determine the freezing point,
the lower part of the thermometer is surrounded with ice shavings,
or freshly-fallen snow ; a better result is thus obtained than
when ice in the a Thermometer. form of small blocks is used.
For a very accurate determination, distilled water contained
in a test tube is frozen round a piece of copper rod, and the
latter having been removed, the thermometer bulb is placed in
the aperture so formed, the space between the bulb and the ice
being filled with distilled water. The whole is then placed in
an inverted funnel filled with ice shavings, and left for a space
of half an hour or so. The position of the extremity of the mercury
column may then be marked, and if it is found that no alteration
in its position takes place in about ten minutes, this point
may be taken as the freezing point, or zero of the thermometer.
- In order to facilitate marking the position
of the freezing point, a thin layer of varnish or paraffin wax
may be laid on the stem ; a scratch in this may be subsequently
etched into the glass by means of hydrofluoric acid gas.
-
- EXPT. 4 .... [An experimental determination
of the zero point of a thermometer using procedure above.]
-
- The correct determination of the boiling
point of a thermometer is a matter of greater difficulty. It
is best to mark the
 position of the extremity of
the mercury column when the bulb and stem are surrounded by steam,
at the same time noting the height of the barometer. A correction
can then be calculated, giving the amount by which the graduation
so obtained is removed from the boiling point under standard
atmospheric pressure.
- The apparatus used for the determination
of the boiling point is shown in Fig. 6. It is best to provide
the cork, through which the thermometer is thrust, with a rather
large hole, the thermometer being prevented from slipping through
by a ring cut from a piece of india rubber tubing fitting tightly
on it. The whole of the stem as far up as the extremity of the
mercury column should be surrounded by steam. When the extremity
of the mercury column has attained a position which does not
alter during five or ten minutes, it can be marked by a scratch,
and the barometer immediately read. The thermometer tube may
now be graduated. A coat of paraffin wax having been laid evenly
over the stem, the distance between the fixed points is divided
into 100 equal parts (200 parts if half degree graduations are
employed). The positions of the graduations are marked by scratchs
in the wax. They may be etched into the glass by hydrofluoric
acid, the vapour being used in preference to the liquid.
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