Acidometer

If the liquid becomes denser, i.e., of greater specific gravity, the acidometer rises, so that more of the tube and its scale is exposed above the liquid, and vice versa.

Source: wiktionary

If it shows by the reading of the acidometer, which is left to float in the electrolyte, that its specific gravity has increased from 1,150°, its density when first poured in, to 1,210 or 1,220 toward the end of the estimated number of hours of charging, then it may be taken for granted that charging has been continued as long as necessary.

Source: wiktionary

The strength of the acid can be determined by the use of an acidometer and a glass cylinder. The acidometer is placed in the cylinder and sufficient acid at 60°F. is then added to make the acidometer float.

Source: wiktionary

We think the acidometer a very dangerous instrument in the hands of the inexperienced, as the solution of ammonia is subject to the influence of air so much.

Source: wiktionary