SM151      Nicholson’s Hydrometer     circa 1965     Hydrostatics

  The apparatus consists of a hollow metal cylinder to which is fixed a cone, loaded with lead to bring the centre of gravity below the metacentre.   This ensures that the hydrometer floats vertically.   On top is a stem terminated by a pan in which is placed the substance whose specific gravity is to be measured.   On the stem is a reference mark.

  In use, masses are placed in the pan to make the reference mark attain the level of the water.   Let the total mass placed in the pan be w.   Now, the substance whose specific gravity is to be determined is placed in the pan and masses are removed so as to bring the reference mark back to the surface.   Let the masses on the pan now be w’.   The weight of the substance placed in the pan is proportional to w – w’.

  In order to find the weight of the substance in water, the sample is transferred from the upper pan to the lower cone that is immersed in water.   Masses are added to the pan until the reference mark is again at the level of the surface.   If the total mass added is m, this is proportional to the weight lost on immersion.   The specific gravity of the solid is equal to the weight in air divided by the weight of water displaced, which is m, the weight of an equal volume of water.

So

  If the substance is lighter than water, the experiment is performed in the same manner, but on immersion is fixed to the lower cone.