Техническое чтение для энергетиков. Бухарова Г.П. - 39 стр.

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41
voltage applied to the instrument terminals. Provided this added resistance does not
vary with temperature, the current flowing through the expansion wire will be
directly proportional to the applied voltage and the expansion will, therefore, be
proportional to the square of the voltage.
When the instrument is to be an ammeter, the expansion wire can be made somewhat
thicker, but if too stout, the instrument will be sluggish in action, as the wire will take
an appreciable time to reach its final temperature. The sizes used in practice are
therefore such as to require only a fraction of an ampere to raise them to their full
temperature. Hence, when a bigger current than this is to be measured, the hot wire
has to be shunted with a resistance of a lower
value, and this reduces the accuracy of the
instrument. As explained previously, the shunt
must be designed so that the current through the
hot wire bears a constant ratio to the total flowing
through the instrument so that the expansion
obtained will be proportional to this current
squared. The use of such a shunt is not very
satisfactory, however, since the resistance of the
hot wire will vary somewhat with different
currents, while that of the shunt will remain
constant.
It will be seen that both in ammeters and
voltmeters, the expansion of the hot wire is
proportional to the square of the quantity being measured, and although the
magnification system modifies this effect somewhat, the result is always a scale very
crowded at the beginning and very open at the end** (see Fig. 10). This may some-
times be a serious disadvantage, since small values cannot be read on a large range
meter. On the other hand, the hot-wire type is particularly suitable for measuring
alternating quantities, since the value of a current which is alternating is defined as
that value which, if flowing continuously, would produce an equal amount of heating.
Hence the hot-wire instrument, or in fact any type obeying what is called a "square
law", will read the same for alternating as for direct currents, whatever the shape of
the alternating wave. This is very useful when measurements are to be made in
circuits in which the current or voltage wave-form departs considerably from the
ideal sinusoidal shape.
Moving-iron Instrument. The electro-magnetic effect of a current is the one chiefly
made use of for measurement purpose. Moving-iron instruments employ this effect.
In principle the moving-iron instrument consists of a fixed coil of wire carrying the
current which magnetizes a small piece of soft iron mounted on the instrument
spindle. In construction there are two varieties – the repulsion type having two pieces
of iron, and the attraction type having only one. In the attraction type of instrument
illustrated in the sketches in Fig. 11 the bobbin С carrying the wire is oblong instead
of circular and has only a narrow slot-shaped opening in its centre. A thin flat piece
of soft iron, A, which is mounted on the instrument spindle is sucked into this slot by
Fig. 10.

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