BRAKES AND DYNAMOMETERS 347 
- friction between a wheel and the rail may be, say, 0°05, while the co- 
efficient of friction between the brake block and the wheel when the wheel 
is not skidding may be, say, 0°08. Let the weight on the wheel be 
10,000 Ibs., and let the braking force on the block be 9000 Ibs. If the 
wheel is skidding the resisting force is 10,000 x 0°05=500 lbs., and 
the work absorbed per foot of travel of train is 500 ft.-lbs. for this 
wheel. If, however, the wheel is not skidding the resisting force is 
9000 x 0°08=720 Ibs., and the work absorbed per foot of travel of 
train is 720 ft.-lbs. for this wheel. But in order that the resistance of 
720 lbs. due to the sliding of the rim of the wheel on the brake block 
may not lock the wheel and make it skid, there must be a resistance to 
sliding of the wheel on the rail of not less than 720 lbs., and this force 
is greater than the 500 lbs. which is the resistance to sliding when the 
wheel skids. Now, when the wheel is rolling, the part of the wheel in 
contact with the rail is for the instant at rest on the rail, and the co. 
efficient of friction between the wheel and the rail before, skidding 
commences may be, say, 0°15, and therefore the resistance before skidding 
commences must be 10,000 x 0°15=1500 lbs., which gives an ample 
margin. 
The coefficients of sliding friction between the wheels and rails when 
the wheels skid, and between the wheels and the brake blocks when the 
wheels roll, are found to vary with the speed, being least at high speeds, 
and they increase as the speed decreases, as shown approximately in the 
following table :— 
Speed of sliding in aoe 
per hour 60 50 40 30 20 10 0 
# between wheels and rails 0°04 | 0°05 | 0:06 | 0°07 | 0:09 | O11 | O15 
0°06 | 0°08 | 0°10 | O13 | O17 | O21 | 0°25 
» between wheels and aga 
blocks 
296. Dynamometers.—A dynamometer is an instrument for measuring 
the effort or torque exerted by or on a machine. The work done in a 
given time by the effort or torque is found by multiplying the effort by 
the distance moved in the given time by the point at which the effort 
acts, or by multiplying the torque by the circular measure of the angle 
described in the given time by the piece on which it acts. 
Dynamometers may be divided into two principal classes, namely, 
absorption dynamometers and transmission dynamometers. In an ab- 
sorption dynamometer the work done by the effort or torque is wasted by 
being converted into heat by means of friction. In a transmission dyna- 
mometer the work done by the effort or torque is transmitted through 
the dynamometer with only a small waste necessary to operate the 
instrument. 
297. Block Brake Dynamometer.— What is generally known as the 
Prony brake is a simple form of absorption dynamometer. In its simplest 
form the Prony brake consists of two blocks of wood clamped together 
with a pulley between them, the pulley being fixed to a revolving shaft ; 
one of the blocks has a lever attached to it, which carries a weight at its 
