A Cavendish balance is described, which has a period of 3.5 min and gives a deflection of the light beam of 3 mm at a distance of 1 m when the position of the outer spheres is changed. This deflection is read with a pair of phototubes which, driving a cathode follower, give a current of 44 µa or more depending on the tube used for the cathode follower. The current can be read with a projection meter. The damping of the balance can be adjusted between critical damping and almost zero.