Техническое чтение для энергетиков. Бухарова Г.П. - 41 стр.

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43
растянутый).
DIRECT-CURRENT METERS
Functions of a Direct Current Meter.– A direct current meter is an instrument
intended for the measurement of electrical quantity in a direct current circuit. There
are two main classes of direct current meters, (1) ampere-hour meters and (11) watt-
hour meters. An ampere-hour meter measures the product of the current in amperes
flowing in a circuit and the time in hours during which the flow is maintained. A
watt-hour meter measures the product of the power in watts and the time in hours
during which the flow of power is maintained.
Direct Current Ampere-hour Meters.– Ampere-hour meters are used by electrical
undertakings for measuring the supply of electricity to domestic and industrial,
consumers. These undertakings are under a statutary obligation to maintain the
voltage at consumers' terminals at a declared value within close limits; assuming that
the supply voltage is maintained at the declared value, an ampere-hour meter can be
calibrated to register in terms of kilowatt-hours at this voltage. This principle is
accepted as satisfactory in most countries where the voltage at consumers' terminals
is maintained within narrow limits of the declared voltage, and since direct current
ampere-hour meters are, in general, more reliable and less costly than direct current
watt-hour meters the practice has much in its favour.
In addition to the foregoing, ampere-hour meters are used for measuring the current
consumption in battery charging, electro-deposition and other electrolytic or
industrial processes and in some instances they exercise a controlling function over
these operations. Many types of ampere-hour meter have been manufactured in the
past, the most important being electrolytic meters and motor meters. Theoretically the
former are capable of very accurate registration but in practice the working results are
not so good as with motor meters, and the latter are preferred by most supply
authorities.
GENERATING AN ELECTRIC CURRENT
The first method used in producing an electric current was chemical in nature.
Credit for its discovery is given to an Italian physician named Aloisio Galvanl
(17.47–1798). One day while engaged in dissecting a frog, Galvanl noticed the leg
muscles contract whenever a nearby electric machine was in operation. Further
investigation showed the same twitching effect
to be obtained by simply connecting
the nerve and muscle of the leg to dissimilar metals. But no such result was obtained
if only one metal was used or if non-conductors were employed. There were
obviously two possible sources of the phenomenon. Either the current was set up at
the junction of the two metals or it was a property of the animal tissues. Galvani
favoured the latter view and in 1791 announced his discovery, attributing the current
to what he called "animal electricity" or as it came to be known, "galvanism".
Galvani is an excellent example of a scientist who behaved most unscientifically with

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