Although the theory of the Tesla motor is now so widely known - though doubtless better known than understood - yet the practical construction of the apparatus has not yet been explained. The following brief description will therefore be read with interest:-

The field-magnet consists of a laminated ring, with internal projections like Paccinotti teeth. It is wound with two independent circuits, A and B; in the motor from which the diagram is taken there are 12 sections in all, and the two coils are wound upon every three sections alternately. The three sections on the right of the figure, and the three on the left, the two being joined in series form circuit A. Similarly, the three upper and the three lower sections form circuit B. The resultant fields of each coil independently are shown by the lines N_{A} S_{A}, N_{B} S_{B}, respectively. But the currents in these two circuits are relatively displaced by ΒΌ phase. Thus, when the current in A is at a maximum, the current in B is at zero, and the resultant field for both coils passes through N_{A} S_{A}; so, also, when B is at a maximum and A is at zero, the resultant is N_{B} S_{B}. But when the two currents are equal the resultant passes through the line N S; the direction of the resultant is, of course, reversed at the moment when the two currents are equal in value but opposite in direction.

The armature is drum-wound, with four coils, as shown, each being short-circuited on itself. The resultant axis passes through N S.