Техническое чтение для энергетиков. Бухарова Г.П. - 37 стр.

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39
to act against some known force or control. Thus, the process of electrical
measurement can be said to consist of a "tug of war"* between two opposing forces –
the operating force, or torque, generated by the electricity which is being measured,
and the controlling force, or torque, which opposes it. When the instrument comes to
rest the pointer indicates the position of stability reached by these two opposite pulls.
Although it does not enter into the actual measurement by influencing this position of
stability, damping is essential to bring the moving system of the instrument to rest in
a reasonably short time. When an electric current flows, it gives rise to various effects
– heating, electrostatic, electro-magnetic and chemical, and any one of these effects
can be utilized to furnish the operating force of a measuring instrument.
* "tug of war"–спортивная игра – «перетягивание на канате»
AMMETERS AND VOLTMETERS
Instrument Connections
The difference between an ammeter and a voltmeter seems to be fundamental, since
the former is connected in series with the circuit and reads the current, whilst the
latter is connected across the circuit and reads the voltage. In practice, however, it
will be found that practically every voltmeter except the electrostatic type is operated
by a flow of current through the instrument, so that the voltmeter is really a form of
ammeter, but reading a very small current at a relatively high voltage.
As an example, take the moving iron type of mechanism which consists of a fixed
coil of wire magnetizing a small piece of iron on the spindle of the instrument. In
order to obtain full deflection it may be necessary for the coil to have an excitation*
of 300 ampere-turns, and if the instrument is to be an ammeter to read up to 10
amperes, this could be obtained by winding the bobbin with thirty turns of a fairly
stout wire. The same mechanism could, however, be employed as a voltmeter by
winding it with a very large number of turns of fine wire, so designed that the
maximum voltage to be measured when applied to the coil sends sufficient current
through it to provide the correct number of ampere-turns.
* for the coil to have an excitation чтобы катушка имела возбуждение; зд. инфинитивный
оборот с предлогом for.
Types of Ammeters and Voltmeters
In order to produce the necessary deflecting torque for the operation of ammeters and
voltmeters the various effects of electric current and of potential – the heating effect,
electrostatic effect, magnetic effect, and electro-magnetic induction effect – are used
leading to a number of different types of instrument. The resulting instruments are
called: (a) hot wire, (b) electrostatic, (c) moving iron, (d) moving coil, and (e)
induction. They are dealt with in some detail in succeeding pages where their
advantages, disadvantages, and specialized characteristics are given. By way of

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