354 APPLIED MECHANICS 
brakes, though convenient in laboratory use, did not become extensively 
employed owing to the skill required in fixing to the motor, and also to 
the small horse-power absorbed for a given size, and cost of apparatus. 
- The great accuracy and convenience of the electrical control of the resist- 
ing torque, and also the cleanliness arising from the use of air instead of 
water to get rid of the heat, was, however, early recognised. 
Morris & Lister, in 1905,* gave the theory of this apparatus, and 
showed how the design might be greatly improved and simplified. They 
showed also that there was a certain. thickness of copper which ought to 
be used on the discs, and that it was just as bad to use too much as too 
little copper. 
Fig. 545 shows the Morris & Lister eddy current brake.t The brake 
may be mounted on its own shaft in a frame, independent of the motor 
to be tested. In some cases, however, especially with electric motors, it 
is simpler to mount the brake direct on the shaft of the motor in place of 
the driving pulley, or, in the case of a petrol motor, in place of the fly- 
wheel. A is a central cast-iron bush, to be secured to the shaft of the 
motor to be tested. On this bush are fitted two ball-bearings B, and 
two strong aluminium spiders C, carrying stout iron dises D, faced with - 
sheet copper on the inner sides, and provided with cooling vanes E on the 
outer sides. On the ball-bearings and between the discs floats a strong 
aluminium casting F, formed for carrying conveniently a number of flat 
iron pole pieces H, arranged in pairs opposite one another. Between each 
pair of poles is a stout iron core K, on which a coil L is slipped. These 
coils are so connected that the poles present alternately magnetised faces 
to each disc. The central casting F has also two strong bosses M and N 
at the ends of its horizontal diameter. Into the boss M is inserted the 
main graduated lever P, on which slides the carrier for the weights W. 
Into the boss N is inserted the short counterpoise lever Q. The long lever 
is sometimes replaced by a short one provided with a hook for a spring 
balance. Further projections S from the central casting F at the top and 
bottom support the outside guards Tand the insulated terminals U, by which 
current is led into the windings from a source of continuous current supply. 
When the magnets are excited, the magnetic flux from each pole is 
compelled to cross the revolving copper disc in order to reach the iron 
dise behind it, and to return by the adjacent magnetic poles. The flux 
has then to cut the other copper dise twice in a similar manner before the 
magnetic circuit is completed. In this way large eddy or Foucault 
currents are generated in the copper discs, which then exert a resisting 
torque, the power corresponding to which is converted into heat in the 
discs. This heat is got rid of by means of the vanes E, which are set: so 
as to induce a strong current of air. 
The dises have to be so supported on the spider as to prevent the 
passage of heat to the arms, and so to the framework of the brake or 
motor, and at the same time to allow them to expand while still keeping 
true. This is done by mica washers and slotted holes in the discs. The 
spiders also must be able to resist the attractive force on the discs, which 
is large, although it diminishes as the speed increases. 
* Journal of the Institution of Electrical Engineers, vol. xxxv. p. 445. 
t Made by Messrs, Morris & Lister, Ltd., Coventry. 
