An Enclosed or Floating Thermometer.—The construction of this type of thermometer is shown by h and i, Fig 11. It is made in the following stages:—A bulb is blown on the drawn-out end of a thin-walled tube as shown by h. A small bulb is blown on the end of a capillary tube, burst, and turned out to form a lip which will rest in the drawn-out part of the thin-walled tube but is just too large to enter the bulb. The capillary tube is introduced and sealed in position, care being taken to expand the joint a little. The thermometer is filled and the top of the capillary tube closed by the use of a small blowpipe flame. A paper scale having the necessary graduations is inserted, and the top of the outer tube is closed as shown by i.
A Maximum and Minimum Thermometer.—If a small dumb-bell-shaped rod of glass or metal is introduced into the capillary tube of a horizontally placed, mercury-filled thermometer in such a position that the rising mercury column will come in contact with it, the rod will be pushed forward. When the mercury falls again the rod will be left behind and thus indicate the maximum temperature attained. If a similar dumb-bell-shaped rod is introduced into an alcohol-filled thermometer and pushed down until it is within the alcohol column, it will be drawn down by surface tension as the column falls; but the rising column will flow passed it without causing any displacement; thus the minimum temperature will be recorded.
Six's combined maximum and minimum thermometer is shown by b, Fig. 11. In this case both maximum and minimum records are obtained from 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 than thirty inches long; the measurement being taken from the junction of the exhaustion branch with the fall-tube to the top of the turned-up end.
Fig. 12
A Macleod Pump.—One form of this is illustrated by b, Fig. 12. It has the advantage that the mercury reservoir may be allowed to become empty without affecting the vacuum in the vessel being exhausted.