How Thermometers are Made

New Zealand Tablet, Volume XXXI, Issue 16, 16 April 1903, Page 13

How Thermometers are Made

The making of a thermometer may be either a delicate scientific operation or one of the simplest tasks of the skilled mechanic, according to the sort of thermometer made (says the London ' Express '). With the extremely sensitive and minutely accurate instruments de* signed for scientific uses great care is taken, and they are kept in stock for months, sometimes years, to be compared with instruments that are known to be trustworthy. But so much time cannot be spent over the comparatively cheap therm ouieteis hi common use, and these are made rapidly, though always carefully. Mercury is generally used for scientific instruments, but most makers prefer alcohol, because it is cheaper. The alcohol is colored red with aniline dye, which does not fade. The thermometer maker buys his glass; tubes in long strips from the glass factories. The glassblower on the premises cuts these tubes to the proper lengths and with his gas-jet and blow-pipe makes the bulb on the lower end. The bulbs are then filled with colored alcohol, and the tubes stand for 24 hours. On the following day another workman holds each bulb in turn over a gasjet until the colored fluid, by its expansion, entirely -fills the tube. It then goes hack into the hands of the glassblower. He closes the upper end and turns the tip backward to make a little glass hook which will help keep the tube in place in the frame. The tubes rest until some hundreds of them, perhaps thousands, are ready. Then the process of gauging begins. There are no marks on the tube, and the first g-uide mark to be made is the freezing point, 32 degrees Fahrenheit. This is found by plunging the bulb into melting snow. No other thermometer is needed for a guide, for melting snow invariably gives the exact freezing point. This is an unfailing test for any thermometer when accuracy may be suspected. But melting snow is not always to be had, and a little machine resembling a sausage grinder is brought into use. This machine shaved a block of ice into particles, which answer the

purpose as well as snow. When the bulbs have been long enough in the melting snow, a workman takes them one -fey one from their bath, seizing each so that his thumbnail marks the exact spot to which the fluid has fallen. Here he makes a scarcely perceptible mark upon the glass with a fine file, and goes on to the next. The tubes, with the freezing point marked on each, now go into the hands of another workman, who plunges the bulb into a vessel filled with water, kept constantly at 64 degrees. Another tiny file scratch is put at 64 degrees.

Then a third workman plunges the bulbs into another

vessel of water, kept constantly at 96 degrees. This 19 marked like the others, and the tube is now supplied With these guide marks, each 32 degrees from the next. With its individuality thus established, the tube goes into the hands of a marker, who fits its bulb and hook into the frame it is to occupy and makes slight scratches on the frame corresponding to the 32 degrees, 64 degrees, and 96 degrees marks on the tube. The frame, whether it be wood, tin, or brass goes to the gaugingrroom, where it is laid upon a steeply-sloping table, marked exactly in the position for a thermometer of that size. A long, straight bar of wood or metal extends diagnally across the table from the lower right hand corner to the upper left hand corner. On the right this rests upon a pivot, and on the left it rests in a ratchet, which lets it ascend or descend only one notch, at a time. Each notch marks the exact distance of two degrees.

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