The force is measured between one leg of a rectangular loop carrying a current and a vertical rod carrying the same current. The rectangular loop is made of 3/l6th inch diameter brass rods. The dimensions of the loop are about 4O by 80 cm. It is suspended from a one inch length of 10 mil steel piano wire. The current enters and leaves loop from mercury pools, which are on the axis of rotation. The current through the loop can be varied up to 50 amperes. The orientation of the loop is set initially so that the plane formed by one leg of the loop and the long vertical rod is perpendicular to the plane of the rectangular loop. The distance between one leg of the loop and the vertical rod can be varied from about 1 cm to over 10 cm. The whole apparatus is oriented so that the plane of the loop is also perpendicular to the earth's magnetic field. The current is turned on in such a direction through the long vertical wire that the loop is repelled. Then the stand holding the loop is rotated about the axis of the loop until this loop has been made to come back to its original position. The angle through which the stand was rotated can be measured. To measure the repulsive force exerted by the current it is necessary to know the dimensions of the loop and the torque constant of supporting wire. This torque constant is determined by measuring the period of the loop and calculating its moment of inertia from its rather simple geometry.