BRAKES AND DYNAMOMETERS 351 
engines, is shown in Fig. 541. This is a design by Prof. W. W. F. Pullen.* 
P is an ordinary cast-iron pulley, 6 inches in diameter, mounted on 
the shaft of the motor to be tested. Small solid-drawn copper tubes C 
are bent to the shape shown. Small plates of brass M having holes 
drilled in them are threaded over the copper tubes, and brazed on to them 
in the positions shown. ‘The copper tubes are placed in position on the 
pulley, and are then embedded in while metal, plaster of Paris being 
used for moulds. The white metal is shown black in the sections, Two 
timber levers T are fitted over the white metal, and are held together by 
two bolts, one of which has a hand-wheel K for a nut, and between this 
hand-wheel and the upper lever there is a helical spring, which enables a 
practically constant pressure to be maintained between the white metal 
and the pulley when the dynamometer is in use. The rubbing surface 
of the pulley is lubricated with oil from the sight-feed lubricator F. 
Water is circulated through the copper tubes, entering at J and leaving 
at-L. The brake is retained in position on the pulley by small wooden 
ear-pieces E. The spring. balance § is useful for measuring small 
variations of torque, but most of the load is put on by dead weights W. 
A dash-pot and any weight required to balance the parts may be con- 
nected at B. This brake easily absorbed 8 horse-power at 1000 revolu- 
tions per minute without undue heating. 
300. Rope Brake Dynamometer.—The simplest and most reliable 
form of absorption dynamometer is probably the rope brake, shown in 
Fig. 542. One, two, or more lengths of rope are passed once round the 
rim of the fly-wheel or the rim of , 
a pulley fixed on the shaft. The 
different lengths of rope are kept 
in position by blocks of wood, as 
shown, the blocks being laced to 
the rope. The upper ends of the 
_ several lengths of rope are united 
and attached to aspring balance B, 
while the other ends are united and 
attached to the weight W. Let 
W = hanging weight, in lbs., in- 
cluding portion of rope, 
hook,etc., hangingfrom A. 
S=tension registered by spring 
balance, less the weight — 
of the rope, etc., between ~ 
A and the balance, in Ibs. 
R=effective radius of wheel 
= nominal radius of wheel 
+ radius of rope, in feet. 
N=number of revolutions of 
wheel per minute. Fig. 542. 
The effective resistance at radius R is W —§, and the brake horse- 
: 2r7RN(W —S) 
power is therefore = 33000 
* Transactions of the Civil and Mechanical Engineers Society (London), 1907. 
