356 APPLIED MECHANICS 
By means of a special regulator, or other suitable regulating appliance, 
the exciting current can be increased until the lever floats, the weights 
having been previously placed to correspond with the required torque. The 
* amount of electric power required for exciting the magnets is quite small. 
Thus a torque of 73 ft.-lb., which gives 14 horse-power at 1000 revolutions 
per minute, requires less than + kilowatt in the coils. The exciting 
current need not be measured. 
The power is computed by means of the formula used for the Prony 
2rW RN 
brake, namely, B.H.P. = “33000 ° 
A good feature of the eddy current brake is that the resisting torque 
is practically constant over a considerable range of speed (10 per cent. 
above or below normal). Hence when testing petrol or other motors in 
which the effort fluctuates, the lever does not oscillate but floats steadily, 
regardless of periodic fluctuations of speed. This constancy of torque 
at or near its rated speed occurs in a similar way to the maximum 
torque in an induction motor, and constitutes not the least of the 
advantages of this convenient and accurate type of absorption brake 
dynamometer. 
A small amount of power is used in overcoming the air resistance 
at the vanes E; a portion of this resistance is communicated to the 
guards T on the floating element, but there remains a certain amount 
which is not communicated to the floating element, and is therefore not 
measured, but, if necessary, this may be allowed for by using a constant 
determined by experiment. The unmeasured resistance is, however, 
only a fraction of 1 per cent. of the total resistance. 
303. Epicyclic-Train Dynamometer.—One form of transmission 
dynamometer is shown in Fig. 546. AB is a lever, which may turn round 
the fixed axle CD. Mounted on the lever, and turning freely on it, are 
two equal bevel wheels E and F, which gear with two equal bevel wheels 
H and K, mounted on the axle, 
and turning freely on it. A 
wheel or pulley is secured to 
the boss L of the wheel H, 
and another wheel or pulley is 
secured to the boss M of the 
wheel K. The torque to be 
measured is transmitted from 
L to M, or from M to L through 
the wheels E and F. 
Let P be the effort exerted 
by the teeth of the wheel H on 
the teeth of the wheel E at a 
radius 7 from the axis of CD, 
and suppose P to act down- 
wards. There will be an equal Fre. 846 
effort P at radius 7 from the axis tes 
of CD acting upwards on F from H. The torque on H is therefore 2Pr. 
The wheel E in driving K will cause the latter to exert a downward 
force P at radius 7 from the axis of CD, and the wheel F in driving K 
will cause the latter to exert an upward force P at radius 7 from the axis 
