##### 4.1 Principles

The lever-arm rule is so called since it is based on the same principle of the seesaw moment-arm balance.

To have the equilibrium it must be:

(weight F_{1}) * distance (F_{1}M) = (weight F_{2}) * distance (MF_{2})

The lever-arm rule can be conveniently applied to mixing calculation on enthalpy-composition and on ternary diagrams for extraction.

In this last case the lever-arm rule is used to easily solve overall mass balances for two mixing streams.

By doing the overall mass balance, it can be seen that F_{1}, F_{2} and M lye on the same straight line and the exact position of the mixing point M can be determined from the mass ratio of the two mixing streams.

The mass of the these two streams F_{1}, F_{2} is then proportional to the distance of M respectively from the point representing the other stream (MF_{2}, MF_{1}).