BRAKES AND DYNAMOMETERS 353 
brake as made by Mr. W. G. Walker, and which the writer has used sue- 
cessfully since the beginning of 1905 for testing petrol motors. 
With the exception of the plates E and F, which are made of alu- 
minium, all the parts are made of steel. The arms AB and CD, of 
rectangular cross section, are clamped to the shaft of the motor by two 
bolts, as shown. The plates E and F are bolted to brackets H and K 
respectively, which are carried by the arms. The plates are equally dis- 
tant from the axis of the shaft, but the distance may be varied according 
to the speed and power of the motor. Different sizes of plates may also 
be used. In the brake used by the author there are three sets of plates, 
the smallest being 6 inches x 6 inches, the intermediate size 84 inches x 84 
inches, and the largest 17 inches x 84 inches. With the smallest plates 
the distance of the outer edges from the axis of the shaft may be varied 
from 8 inches to 15 inches ; with the intermediate size plates this distance 
is from 10} inches to 174 inches ; and for the largest size, 19 inches to 
26 inches. 
The resistance is, of course, the pressure of the air on the plates and 
exposed parts of the arms, and this produces a pure torque, and there is 
no bending action on the shaft, except that due to the weight of the 
instrument, which is only about 93 lbs. with the smallest plates, and 
14} lbs. with the largest. The parts are also perfectly balanced. 
Once the instrument is calibrated it is only necessary to know the 
speed to determine the horse-power, since the power to drive the fan is 
proportional to the cube of the speed. For example, when the inter- 
mediate size plates are used, and the bracket pins are in the sixth holes 
from the inside, the brake horse-power is given by the formula, 
B.H.P. = 0:000,000, 0038N° = 38 x 10°N®, 
where N is the speed in revolutions per minute. . 
The resistance to the motion of the plates is proportional to the density 
and viscosity of the air, Now the density is proportional to the pressure, 
and inversely proportional to the absolute temperature, while the viscosity, 
according to some authorities, is proportional to the absolute temperature. 
Hence the resistance varies with the pressure only, and if the coefficient 
¢ in the equation B.H.P. =cN? is obtained experimentally when the height 
of the barometer is h, then if, when a test is made, the height of the 
barometer is h’, the coefficient ¢ should be multiplied by h’/h. 
This dynamometer may be run for any length of time, as there is no 
heating effect on the instrument, the heat being carried away by the 
circulating air. 
The dynamometer having the dimensions given above may be used 
for powers up to 20 horse-power. 
302. Eddy Current Brake Dynamometer.—In the eddy current 
brake dynamometer the resisting torque is obtained without actual con- 
tact between the revolving and the floating elements. A system of field 
Page, with alternate poles, is mounted on the floating portion of the 
brake, while the motor under test drives one or two copper dises past the 
pole faces, whose magnetic flux induces very large circulating currents, 
which, by their magnetic action, tend to retard the motor and absorb its 
energy in heating the discs. 
Brakes on this principle were constructed by Pasqualini in 1892, by 
Grau in 1900, by Siemens and Halske in 1901, and by others. These 
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