The whole of the work is converted into heat at the rubbing surfaces 
between the rope and the wheel. For small powers or for short trials 
the air in contact with the revolving wheel will carry away enough heat 
- to’ keep the wheel sufficiently cool. For larger powers and long trials, 
however, it is necessary to cool the wheel rim with water. For water 
cooling it is best to make the rim of the wheel of channel section, as shown 
in Fig. 543. The 
water is held in by 
centrifrugal force 
so long as the speed 
of rotation is not 
less than a certain 
critical speed. The Fig. 543. 
centrifrugal force of a small mass of water weighing w lbs. at a radius 
2 
R feet and revolving at N revolutions per minute is F= aaa 
When this mass of water is in its highest position the resultant force 
holding it to the rim of the wheel is F —w, and the minimum speed at 
which the water will remain in contact with the wheel in its highest 
352 APPLIED MECHANICS 
position is found by putting F=w, then N= i VE 
During a long trial it is necessary to renew the cooling water. This 
may be done by using two pipes, as shown in Fig. 543. One pipe D 
supplies cold water to the channel in the rim, and the other E scoops 
water out and discharges it. Just before stopping a trial the water 
supply is cut off, and the pipe E is turned over slightly so as to nearly 
touch the bottom of the channel and collect and discharge sufficient 
water to prevent an overflow from the channel when the wheel stops. 
The collecting end of the pipe E is flattened out so as to present a 
narrow slit to the water. 
301. Fan Brake Dynamometer.—One of the simplest and most con- 
venient of dynamometers for testing the output of small high-speed motors 
is a simple form of fan. This was first used by M. Renard, whose design 
for small powers consisted of a rectangular bar of ash, to which were 
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Fig. 544. 
bolted two rectangular aluminium plates, the wooden bar being mounted 
on the shaft of the motor at right angles to its axis. Fig. 544 shows the 
