Two flat coils, one of thick wire, the other of thinner wire, represented diagrammatically at A A and B B of Fig. 1, were set at right angles to one another. Into the first was brought a current from the primary of a Gaulard's transformer, and into the second the current from the secondary, with more or less non-inductive resistance. In the central space was suspended a small hollow closed cylinder of copper. If the current was turned on in one only of the two windings the cylinder remained immovable, but on turning on the second current it at once began to rotate. The sense of the rotation could be reversed by simply changing, with a reversing-switch, the connections of the second coil. The same results were found to follow when a cylinder of iron was substituted for that of copper. A laminated iron cylinder built up of insulated disks also turned. Then followed suggestions for constructing alternate current motors on this principle but of modified form; for, as Professor Ferraris remarked, it was evident that a motor thus made could not have any importance as a means of industrial transformation of power. He therefore designed a larger model, having as its rotating part a copper cylinder weighing 10 lbs, having a length of 18 cm, and a diameter of 8,9 cm, borne on a horizontal shaft 1 cm in diameter. It was surrounded by two sets of coils A A and B B at right angles to one another, as in the Fig. 2. It was, however, of but small power. Ferraris discussed the elementary theory of the apparatus, pointing out that the inductive action would be proportional to the slip, that is to say to the difference between the angular velocity of the magnetic field and that of the rotating cylinder, that the induced current in the rotating metal would also be proportional to this; and that the power of the motor is proportional jointly to the slip and to the velocity of the rotating part. Ferraris also suggested measuring instruments for alternate currents based on this principle. Lastly he succeeded in producing rotation in a mass of mercury placed in a vessel in the rotatory field. In 1894 Ferraris published another discussion of the theory of these motors.