44. Hydrometer As a Means of Measuring Specific Gravity. -A hydrometer is a convenient instrument for measuring the specific gravity of a liquid. The principle upon which the hydrometer is based is as follows:-If a body is floated in a liquid it sinks until the weight of the liquid displaced is exactly equal to the weight of the body. If the liquid is dense (has a high specific gravity), the body will not sink as far as it would in a liquid of lower specific gravity. An example of this is shown when a piece of wood is floated in water. The wood will sink to an appreciable depth in water, but the depth to which it would sink in mercury (quicksilver) is hardly noticeable. In fact, mercury will give a greater buoyancy to iron than water will give to ordinary wood. Therefore the depth to which a body sinks in a liquid is an indication of the. specific gravity of the liquid. Such a body, calibrated to read the specific gravity of a liquid, is called a hydrometer.

Figure 38 is a drawing of a syringe hydrometer of the type used to measure the specific gravity of the storage battery electrolyte. It consists of a glass float (hydrometer) in a glass barrel. At the top of the barrel is a rubber bulb and at the bottom a rubber nozzle. The float consists of a small glass tube sealed at both ends to prevent the liquid from entering it. In the lower end of the tube are lead shot-weights, held in place by sealing wax, to weight that end of the float down so that it will float in an upright position. The upper part of the tube is a thin neck graduated in lines which are numbered to indicate the specific gravity of the liquid tested. Because the neck of the tube is of small diameter it is necessary for the tube to sink, or rise, considerably for each slight change of specific gravity of the liquid tested.

45. Using a Syringe Hydrometer to Determine the Specific Gravity of Battery Electrolyte. -When using a syringe hydrometer to determine the specific gravity of the electrolyte of a storage battery, first insert the nozzle of the syringe into the cell and squeeze the bulb, and then slowly release the bulb to draw up just enough electrolyte from the cell to freely float the hydrometer. Be sure that no large air bubbles cling to the sides of the hydrometer, and that it does not stick to the glass barrel. Shake the instrument a little to make sure that the glass float is entirely released when the reading is taken. The reading on the stem of the hydrometer at the surface of the liquid is the specific gravity of the electrolyte. Return the electrolyte to the cell from which it was taken, making sure that all the electrolyte is out of the syringe before removing it from the cell. It is a good plan to keep the nozzle of the syringe in the cell when a specific gravity reading is being taken, because electrolyte is almost sure to drip out and some of it will be lost. Besides, the electrolyte dropped on the top of the battery or other objects will cause corrosion unless cleaned off promptly.

Never measure the specific gravity of a cell immediately after adding water. Such a reading would not give the specific gravity of the electrolyte in the cell because the acid at the top of the cell would be diluted. After adding water, charge the battery until it gases freely before taking the next specific gravity reading.

A cheap hydrometer may be in error by as much as 30 points and should not be used.

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