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Home Measurement of Mass and Specific Gravity Nicholson's Hydrometer  
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Nicholson's Hydrometer
This instrument is used (1) to determine the specific gravity of small solids which can be immersed in water; (2) to determine the specific gravity of a liquid. (1) To find the Specific Gravity of a Solid. Taking care that no airbubbles adhere to it, place the hydrometer in a tall vessel of distilled water recently boiled, and put weights on the upper cup until it just sinks to the mark on the stem.
It will generally be found that with given weights on the cup the hydrometer will rest in any position between certain limits; that there is no one definite position of flotation, but many. The limits will be closer together and the experiment more accurate if the surface of the instrument, especially that of the stem, be thoroughly clean and free from grease. It is well therefore carefully to rub the stem and upper part of the bulb with some cottonwool soaked in methylated spirit. Suppose now it is floating with the mark on the stem just below the surface. Take off some weights until the mark just rises past the surface; let the weights then on be 8.34 grammes. Put on weights until the mark just sinks below the surface, and then let the weight be 8.35 grammes. Do this several times, and take the mean as the weight required to sink the mark to the surface. Let the mean be 8.345 grammes. Remove the weights and put the solid in the upper cup. Then add weights until the mark again just comes to the surface, estimating the weight required as before. Let this be 2.539 grammes. The weight of the solid in air is the difference between these, or 5.806 grammes. Now put the solid in the lower cup^{1} and weights in the upper one until the mark sinks to the surface. Estimate this as before. Let the mean of the weights be 5.462 grammes. The difference between this and the weight 8.345, put on originally to sink the hydrometer, gives the weight in water. Thus, the weight in water = 2.883 grammes. And the weight of water displaced = weight in air  weight in water = 2.923 grammes. The specific gravity, therefore, referred to water at the temperature of experiment
To determine the true specific gravity  water at 4° C. being taken as the standard  we must multiply this number by the specific gravity of the water at the time of the experiment. This may be taken from the table (32), if we know the temperature. Thus, we must observe the temperature of the water at the time of the experiment. Let it be 15°. Then the specific gravity required
(2) To determine the Specific Gravity of a Liquid. Let the weight of the instrument itself be 11.265 grammes. This must be determined by weighing it in a balance. Place it in the water, and put weights on the upper pan until it just floats up to the mark on the stem. Let the weight be 8.345 grammes. This of course must be estimated as in experiment (1). The sum of these two weights is the weight of a volume of water equal to that of the instrument up to the mark on the stem. Thus, the weight of this volume of water is 19.610 grammes. Now place the instrument in the liquid and add weights till the mark is just in the surface. Let the weight be 9.875 grammes. Then the weight of the volume of liquid displaced is
The specific gravity of the liquid referred to water at the temperature of the experiment is therefore
Let the temperature of the water be 15°C.; that of the liquid 11.5°C. Then the specific gravity of liquid at 11.5°C. is 1.078 x 0.99917 = 1.077. Experiments
(1) Determine the specific gravity of sulphur by Nicholson's Hydrometer. Enter results thus: (a) Specific gravity of sulphur.
(b) Specific gravity of salt solution.


Home Measurement of Mass and Specific Gravity Nicholson's Hydrometer 