The design of a balance for measurement over the range from 4.2 to 300°K of the static magnetic susceptibility of weakly magnetic substances is described. The balance is a Curie‐Che´neveau type torsion balance which incorporates photocompensation to achieve a null measuring instrument. The maximum sensitivity of the balance is approximately 10−3dyne and performance with less than 1% error is possible for magnetic susceptibility values in the range from 10−5to 10−8emu/g (cgs units). Some typical susceptibility data obtained with the aid of this balance are presented.