This instrument consists essentially of an iron drum with an annular recess, in which are situated one or more coils of wire, the ends being connected to pairs of corresponding slip-rings. The drum is mounted on ball bearings, and its periphery, which is shod with cast-iron rings, is machined to run true to 0.0001 in. A small iron or steel shoe fits accurately the curvature of the rings. When a current passes through the coil the drum is magnetized, and this causes the shoe to be pressed on the rings with considerable force. Thus, if the drum is revolved, a pull is required to cause the shoe to slide relatively to the rings. The magnitude of the tangential pull thus obtained is many times greater than that calculated from the product of pressure due to magnetic attraction, using the formulaB2A/8π, and an assumed coefficient of friction μ = ⅓. Subsequent experiments show, however, that the value of fi is about 0.6. The ratio of experimental pull to calculated pull (takingμas 0.6) depends on the flux density at the shoe contact, and may exceed 50. Thus the operation of the device appears to depend on some form of cohesive action incited by magnetism. The present paper deals with this phenomenon, but more particularly, with its application to electric relays, siphon recorders for line and radio-telegraphy, and transmitting keys for radio-telegraphy. The electric circuits for single-current, double-current and valve-circuit working are described in detail.