Brake, FRICTION

or PRONY. Dynamometer for measuring the power given off by a motor through a revolving shaft. A brake-shoe or band is pre ed against the circumference of a shaft or pulley by means of bolts enveloping the pulley, and the number of pounds nece ary to hold that shoe or band from moving is the amount of friction on that brake. The product of that number of pounds into the number of feet per minute through which the point of application of the weighed load would have moved had it been free at that speed of revolution, gives the compound unit of foot-pounds per minute. The horsepower is derived by dividing the measured foot-pounds in one minute by 33,000, which is the number of foot-pounds in one horse-power. Brake Tests. Those made by Captain Galton show that coefficient of friction between the shoe and the wheel decreases with the speed. A formula showing the rate of decrease is IMG:content-0602.jpg:[graphic] IMG:content-0603.jpg:[blocks in formation] in which f is the coefficient of friction and vis the speed of the train in miles per hour.