EDDY-CURRENT BRAKE 



207 



The zero position of the sliding weight W is that in which it just 

 balances the weight of the magnet pole-pieces on the other side of 



BRA.KE DISC 



SLIDIN& WEIGHT 



GRADUATED ROD 



OF MOTOR-vD 



2 



BRAKE DISC 



(*) 



Fio. 135. Grau'a Eddy-current Brake. 



the knife-edge when the motor is at rest. The force F due to eddy 

 currents is given by the equation 



d being the displacement of W from its zero position, and a the 

 distance of the middle line of the pole-pieces from the knife-edge. 

 The corresponding useful torque of the motor is given by F . /, where 

 / is the distance of the middle line of the pole-pieces from the centre 

 of the shaft. By suitably varying the exciting current of the brake 

 magnet, the lever may be kept absolutely steady. 



An extremely convenient form of eddy-current brake has recently 

 been described by Messrs. D. K. Morris and G. A. Lister.* The 

 entire brake is mounted on the motor shaft, replacing the pulley. 

 The essential parts of the brake are shown in Fig. 136. There are 

 two brake rings of copper, each of which is supported by a steel disc 

 and a number of steel arms, riveted to a hub. Each hub is keyed to 

 a bush, which in its turn is keyed to an inner bush, the latter being 

 finally keyed to the motor spindle. The reason for providing an 

 inner bush is that by having several inner bushes of different internal 



* The Electrician, vol. Iv. p. 88 (1905). 



