The present paper presents a study of this hydrometer and makes several suggestions: (1) Care must be exercised in the choice of ferrous material for the core and of the weight of that core. Permalloy is recommended. (2) Unless permalloy is used, a mixture of d.c. and a.c. is recommended for producing the magnetic pulls, only the d.c. being measured. (3) The equations are used to give hints for calibration and design.