substances such as rubber, oil, ceramics, and glass usually make good insulators; metals usually make
good conductors. The resistance of an insulator is expressed in megohms and is measured by an
instrument called a megger.
1.17. CURRENT
An ampere is the rate of flow of electric current. Technically, it is that flow of current which
deposits a specific weight of silver from a standard silver nitrate solution under prescribed conditions in
1 second.
1.18. OHM'S LAW
George Simon Ohm, a German physicist, created the electrical formula known as Ohm's law in
1827. Ohm's law states the relationship between voltage, resistance, and current: in a simple closed
circuit of fixed resistance the current in the circuit is equal to the voltage divided by the resistance.
Resistance is represented by R, voltage by E, and current by I; and the formula is I =
1.19. SYMBOLS
An electrician uses standardized symbols to represent the various parts of a circuit. Those most
frequently used in locomotive circuits are shown in figure 1.11. The symbols represent basic parts of
apparatus instead of a separate symbol for each piece of equipment. For example, the coil of a ground
relay, a wheel slip relay, and a field shunting relay are always represented by the same symbol for an
operating coil and the contacts are always represented by the same symbol for contacts. To identify
separate pieces of equipment, the drawing symbols are accompanied by letters, such as GR for ground
relay or WS for wheel slip relay. In an actual wiring diagram, the location of parts of the circuit are
shown in their true relationship; in a schematic wiring diagram, all the parts are shown but not
necessarily in their true locations.
1.20. SCHEMATIC WIRING DIAGRAM
The schematic wiring diagram for a locomotive is very much like a road map. The lines, used to
show electric circuits (highways), and symbols, used to show the devices in the circuits (cities), form an
electrical map to help in tracing out circuits and locating troubles on a locomotive. Unlike an
automobile trip, which
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