Ohm’s law. The law that explains the relationship between the flow of electrons (current), electrical pressure (voltage), and the opposition to flow (resistance). According to Ohm’s law, the amount of current flowing in a circuit is directly proportional to the circuit voltage and inversely proportional to the circuit resistance.
ohmmeter. An electrical measuring instrument that measures the resistance in a circuit or component and shows its value in ohms. An ohmmeter determines the value of an unknown resistance by measuring the current flowing through it when the voltage across it is known.
ohms-per-volt (voltmeter sensitivity). A measure of the sensitivity of a voltmeter, found by dividing one by the amount of current (in amperes) needed to deflect the pointer of the meter full scale.
For example, a meter that requires one milliamp (0.001 amp) of current for full-scale pointer deflection has a sensitivity of 1,000 ohms per volt.
The greater the ohms-per-volt, the more sensitive the meter.
oil analysis. A method of measuring the contents in parts per million of various chemical elements in a sample of oil. A sample of the oil is burned in an electric arc, and the resulting light is analyzed with a spectroscope which identifies the chemical elements in the oil and gives an indication of the amount of each element.
oil can (sheet metal problem). A condition in which a flat piece of sheet metal is stretched so it snaps in and out between rows of rivets, in the same way the bottom of an oil can snaps in and out.
oil circuit breaker. A protection device for electrical circuits that carry large amounts of current. The housing containing the current-carrying contacts is filled with oil which quenches the arc that forms as the contacts open. Quenching the arc prevents damage to the contacts when they open a circuit carrying a large amount of current.
oil control ring (reciprocating engine component). One of the cast-iron rings installed in grooves around a piston. The oil control ring is installed in the groove below the compression rings (top rings), and it is usually made in several sections.
The oil control ring controls the amount of oil allowed to flow between the piston and the cylinder wall. Excess oil is drained back into the crankcase through holes drilled through the piston in the ring groove.
oil-cooled transformer (electrical transformer). An electrical transformer whose core and coils are immersed in oil. The oil circulates through the transformer and carries heat from the core and windings to the housing. Air flowing around the housing removes this heat. The housings for small oil-cooled transformers have a smooth outer surface, but the housing of larger units is sometimes finned to assist in carrying away heat.
oil cooler (reciprocating engine component). An air-oil heat exchanger, used to remove excess heat from the lubricating oil. If the oil is too hot as it leaves the engine, it flows through the oil cooler, where it gives up the excess heat to the air flowing through or around the tubes in the cooler. The cooled oil then returns to the engine oil sump or to the oil tank.
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