Техническое чтение для энергетиков. Бухарова Г.П. - 38 стр.

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40
introduction*, however, it may be well to point out that the various types are usable
on both d. c. and a. c. circuits with two exceptions, namely, the permanent-magnet,
moving-coil instrument which can be used only with d. c., and the induction instru-
ments which are limited to a. c. operation. For general purposes the moving-iron
instruments are by far the most commonly used, while for d. c. work the permanent-
magnet, moving-coil instrument is the best, being specially suitable for use with
shunts and multipliers for multi-range purposes. The other types are uncommon for
general work, although each has advantages under certain conditions. Thus the order
in which the instruments are described below is not that of relative importance, but
merely such as to fit in with the general arrangement of the presentation.
Hot Wire.– When an electric current flows along a conductor the latter becomes
heated, the heating effect being simple to use for measurement purposes. The heat
produced per second in a conductor of any given resistance is proportional to the
square of the current, and the conductor will, therefore, be heated up to the point at
which it radiates each second all the heat which the current is producing. Its
temperature rise above the surroundings will then be proportional to the square of the
current flowing, and it is only necessary to measure this temperature rise in order to
get a measure of the amount of the current.
The simplest way of measuring the temperature rise is by means of the linear
expansion which it causes in the wire carrying the current, and hot-wire instruments
work on this principle. Unfortunately the expansion is extremely slight, being of the
order of one hundred thousandth of the length for each degree centigrade, and it
therefore becomes necessary to magnify the movement many times before trans-
mitting it to the pointer of the instrument. This complicates what would otherwise be
a very simple type of mechanism, and it also introduces errors due to the stretch of
the various wires employed or to the interplay of the magnifying mechanism. Other
errors may be introduced due to the expansion of the baseplate of the instrument and
for these reasons the hot-wire instrument frequently exhibits a zero error and is
usually only of second-grade accuracy.
Fig. 10 shows diagrammatically a hot-wire instrument employing what is called the
double-sag method of magnifying the expansion movement. The current to be
measured, or some definite fraction of it, passes through the expansion wire E, which
is made of platinum-iridium or platinum-silver, either of which alloys will stand a
high temperature without oxidization. When this wire is heated and expands, its sag is
taken up by a phosphor bronze wire P, and the sag in this is taken up by a silk fibre F,
which is kept taut by a small spring S. This silk fibre passes round a small pulley
mounted on the instrument spindle and a very minute expansion in the hot wire E
causes a considerable movement of the spindle, and is shown by the pointer mounted
on it.
When the instrument is required to be a voltmeter the expansion wire is connected in
series with a high resistance whose value will depend upon the voltage range which is
required. The current necessary to operate the hot wire is usually of the order 0.1 to
0.2 amp., and as the resistance of the wire itself is only 10 ohms or so, it will be seen
that except on very low ranges the added resistance absorbs the greater part of the

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