The bifilar magnetometer has been employed in all the magnetical observations, with an exception or two; its theory has been stated generally, but no careful examination of the errors of the instrument, as deduced from experience, has been published. The author has attempted to supply this deficiency.

In the theory of the instrument, the proper elasticity of the wires has been neglected, as of little importance. The author shows that the wires receive a twist in the adjustment, which puts the upper suspension-points into a different plane from the lower ones; that the force so produced enters into the equation of equilibrium in a marked manner. The theoretical conclusion is verified by experiments on the torsion force of the wires.