552 JAMES CLEftK MAXWELL. 



current. A second bevel wheel D turns loosely on the arm 

 C D, which is one of four arms (of which only two are shown 

 in the figure) forming a cross, which can turn freely on the 

 central shaft at C. Sliding weights M M', etc., can be 

 fixed in any desired position on these arms so as to alter 

 the moment of inertia of the cross, which is the differential 

 piece in the mechanism. A third bevel wheel B is keyed 

 to the same hollow shaft with the wheel S, which is similar 

 to P, and the rotation of the piece B S represents the current 

 in the secondary circuit. As the shaft B S is hollow, and 

 rides loosely on the shaft A C, the wheels A and B can 

 turn quite independently of one another, except in so far 

 as they are connected by the wheel D. P' is an index 

 attached to the interior shaft and turning with P. A loop 

 of string is hung over each of the wheels P and S, and 

 carries a small weight. These strings act as friction brakes 

 to the wheels, and the friction represents the resistance of 

 the primary and secondary circuits respectively. The 

 moment of inertia of the loaded cross, or differential piece, 

 represents the moments of inertia of the cells which con- 

 stitute the molecular vortices in the dielectric. Its kinetic 

 energy when rotating represents the energy of the vortices, 

 and its angular momentum is proportional to the electro- 

 magnetic momentum of the system. The moments of inertia 

 of the other portions of the mechanism are very small com- 

 pared with that of the loaded cross. The motion of the cross 

 and the wheel D is impeded by as little friction as possible. 

 Suppose that the wheel P is made to revolve, repre- 

 senting a current in the primary wire ; the heavy cross will 

 not at first move, but the wheel D will revolve and com- 

 municate the motion to B, which, with S, will rotate in the 

 direction opposite to that of P, representing a current in the 

 secondary circuit opposite in direction to that in the 

 primary. But the motion of S is resisted by the friction 

 brake, and a finite force must therefore be exerted by D on 

 B to drive it. The reaction of B, together with "the force 

 exerted by A, will constantly tend to make the cross revolve 

 in the same direction as P, and the velocity of the cross 



