A Hydrometer is an instrument used to determination of specific gravity of liquid.
As the specific gravity of soil suspension depend upon particle site, a hydrometer can be used for particles size analysis.
Special type of hydrometer with a long stem (Neaks) is used.
The stem is marks from top to bottom, generally in the range of 0.995 to 1.030.
At the time of commencement of sedimentation, the specific gravity of suspension is uniform at all the depth.
When the sedimentation takes place, the layer particle settle more dipper then smaller ones.
This results is non-uniform specific gravity of the suspension at different depth.
The lower layer of the suspension has specific gravity greater then that of the upper layer.
Casagrande has shown that the hydrometer measures the specific gravity of suspension at a point indicated by the center of the immersed volume.
If the volume of the stem is neglected, the center of the immersed volume of the hydrometer is the same as the center of the bulb, thus the hydrometer gives the specific gravity of the suspension at the center of the bulb.