. An elementary book on electricity and magnetism and their applications. rail andwheel may be equal to 30 per cent of the pressure between wheeland rail, whereas if the rail is wet it may be only half thatamount. The addition of sand will increase the friction fromabout 15 per cent to about 25 per cent. This force, however,is available only when the wheels are rolling on the rails; assoon as slipping occurs the friction rapidly decreases. So theforce which opposes the revolution of the wheels, namely, thebrake-shoe friction, must never exceed that which is keepingthe wheels turning, namely, t
. An elementary book on electricity and magnetism and their applications. rail andwheel may be equal to 30 per cent of the pressure between wheeland rail, whereas if the rail is wet it may be only half thatamount. The addition of sand will increase the friction fromabout 15 per cent to about 25 per cent. This force, however,is available only when the wheels are rolling on the rails; assoon as slipping occurs the friction rapidly decreases. So theforce which opposes the revolution of the wheels, namely, thebrake-shoe friction, must never exceed that which is keepingthe wheels turning, namely, the friction between the wheelsand rails. The force which is applied to the brake shoes is nowvery generally furnished by compressed air. There are severalsystems of air brakes which are adapted to various conditionsand kinds of service. All of these systems require a motor-driven air compressor, a reservoir tank, brake cylinder, governor,valves, pressure gauges, and much piping. Such appliancesmake the modern electric street car a very complicated andexpensive Fig. 357. — Street-car controller with the door open. Hi ^^B^^^^^^^^ l^^_^^^^5^BimmHBB^^^ 1TJ .n i eitfl ifil ^ij- ? nils ill | J1 B *iiM ^tHHBaHM^MM^ i»-~ ?- *^h ^^
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
