Conversations on Chemistry, V. 1-2 / In Which the Elements of that Science Are Familiarly Explained and Illustrated by Experiments

by Mrs. Marcet

It looks like a double thermometer reversed, the tube being bent, and having a large bulb at each of its extremities. ([Plate II.] Fig. 2.)

EMILY.

Why do you call it an air thermometer; the tube contains a coloured liquid?

MRS. B.

But observe that the bulbs are filled with air, the liquid being confined to a portion of the tube, and answering only the purpose of showing, by its motion in the tube, the comparative dilatation or contraction of the air within the bulbs, which afford an indication of their relative temperature. Thus if you heat the bulb A, by the warmth of your hand, the fluid will rise towards the bulb B, and the contrary will happen if you reverse the experiment.

But if, on the contrary, both tubes are of the same temperature, as is the case now, the coloured liquid, suffering an equal pressure on each side, no change of level takes place.

CAROLINE.

This instrument appears, indeed, uncommonly delicate. The fluid is set in motion by the mere approach of my hand.

MRS. B.

You must observe, however, that this thermometer cannot indicate the temperature of any particular body, or of the medium in which it is immersed; it serves only to point out the difference of temperature between the two bulbs, when placed under different circumstances. For this reason it has been called differential thermometer. You will see by-and-bye to what particular purposes this instrument applies.