To measure directly the density of a liquid.
It is essentially a balance of asymmetric arms: the shortest ends in an adjustable counterweight which incorporates a needle and the longest is divided into ten equal parts and carries a hook at its end.
The set is completed with an adjustable height base (equipped with a needle that acts as an index to fix the balance position), a glass float (which contains a thermometer inside and it is held by a platinum thread) and a set of weights or reiters of different mass (two large ones which, although different in form and function, have the same weight, and other three smaller ones, whose weights are the tenth, the hundredth and the thousandth of those, respectively).
The liquid, whose density we want to determine, is poured into a test tube and we immerse the float completely in it. Due to the pressure exerted by the liquid, the balance becomes unbalanced and, in order to rebalance it again, it is necessary to place the reiters in different positions marked along the long arm. The position of these weights gives us, applying a correction according to the temperature measured by the thermometer, the value of the density of the liquid.
It was developed by the German pharmacist Karl Friedrich Mohr (1806-1879).