Техническое чтение для энергетиков. Бухарова Г.П. - 40 стр.

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42
magnetic attraction when the current flows, the motion being opposed, in this case,
by the weight W.
Either gravity or spring control can be employed on
moving-iron instruments, and damping is usually by
means of an air dash-pot. This latter is clearly
shown in both of the above figures, where it takes
the form of a light piston (P. Fig. 11), fixed to the
spindle in such a way that it describes an arc of a
circle within a curved cylinder whose walls it nearly
touches.
Moving-Coil Instruments.– In moving-iron
instruments the magnetic field is furnished by a
fixed coil of wire carrying the current, and a piece
of soft iron moves in this field. In the moving coil
instruments now to be considered, this arrangement
is reversed; the field is provided by a fixed
permanent magnet, whilst a coil carrying the current
moves within this constant field. Fig. 12. shows the
general arrangement
of a modern moving-
coil permanent-
magnet instrument.
It will be seen that the instrument consists of a
permanent magnet M, having two pole pieces, P. In
between these pieces a circular iron core / is fixed by
means of the bracket B, the latter being made of some
non-magnetic material such as brass. This leaves a
small annular air-gap in which the coil F is free to
turn. This coil consists of a light aluminium
framework carrying a number or turns of fine silk-
insulated copper wire.
When the current flows round this coil the latter tends
to turn in the magnetic field provided by the permanent magnet, and its motion is
opposed by the spiral hair spring C, of phosphor bronze, which also serves to lead the
current into the coil. Usually there is a second control spring fitted at the other end of
the instrument and wound in the opposite direction so as to neutralize temperature
variations in the spring. This will then serve also to lead the current out of the moving
coil, but in some cases a flexible metallic ligament serves this purpose and exerts
no appreciable restriction on the movements of the coil.
* by way of introduction в качестве предисловия; by way of – составной предлог ради, с целью;
через
** a scale very crowded at the beginning and very open at the end шкала очень сжата в начале и
очень растянута в конце. В применении к делениям шкалы Past Participle от глагола (to)
crowd (букв, тесниться, толпиться имеет значение сжатый, а прилагательное open –
Fig. 11.
Fig. 12. Sketch of Moving-
coil Instrument

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