The description below is taken from "Practical Electricity" which was printed in 1891.

This describes almost exactly my own version of the voltmeter.

 

Cardew's Voltmeter

This voltmeter, designed
by Captain Cardew, R.E., differs from all the instruments
previously described in that the heating and not the
magnetic action of a current is employed, and the elevation
of temperature of the conductor is measured by
its expansion. The conductor consists, in the newest
form of the instrument, the back of which is seen in
Fig. 167, of about thirteen feet of platinum - silver
wire 0.0025 inch in diameter. This wire, which is fixed
at one end to a screw A, passes, at the top of the instrument,
over a pulley P1, made of bone so as to be an
insulator, then down under a small bone pulley p1, then
up again over a bone pulley P2, and lastly is fastened to
a screw B. The pieces of brass into which the screws A
and B are fastened, are connected with the terminals T1,
and T2, and on a P. D. being set up between these terminals
a current flows through the stretched wire, the
strength of which depends on the P. D. maintained between
the terminals of the voltmeter, and on the resistance
of the wire. The wire becomes hot and expands,
and as it is very thin, it very quickly acquires the
temperature corresponding with the particular current
passing through it. The support carrying the little
pulley p1, is pulled down by a thread wrapped round the
grooved wheel W and fastened to the spring S1; hence
when the wire lengthens, and the little pulley p1 descends,
the wheel W is turned. The staff (or little shaft) carrying
the wheel W also carries a toothed wheel L, geared
into a small pinion M, hence when, by a slight lengthening
of the wire, W is turned through a small angle, the pinion
turns through a large one. On the farther end of the
staff carrying the pinion there is fixed, in the front of
the instrument, a pointer moving over a dial graduated
in volts, the back of which is seen in the figure; consequently
the pointer is caused to move right round the
scale by a comparatively small descent of the pulley p1.
It will be observed that the pull of the spring S1 is
balanced by twice the tension in the stretched wire, and
that the descent of the pulley p1 is due to the expansion
of only half the total length of wire employed, that is,
the expansion of only about six feet six inches of wire.
The advantage, however, of using a long wire, fixed in
the way shown, instead of a wire half as long, and of
twice the sectional area, which would enable the same
spring S1 to be used and cause the same motion of the
pointer for the same elevation of temperature, is that the
fine wire heats and cools much more rapidly than the
thicker one, and so makes the voltmeter much more deadbeat.
If the P. D. to be measured is between 30 and
120 volts, the stretched wire alone may be used, but for
larger P. Ds., an extra resistance is
added, and the terminals of the voltmeter are now T1
and T3. If the extra resistance be equal to that of the
voltmeter itself, not merely when the wires are cold, but
also when they are heated by the passage of the current,
the readings on the scale will correspond with exactly
twice the number of volts ; or a double scale can be employed,
the numbers on the one being twice the corresponding
ones on the other. To insure the resistance of the
added wire being always exactly equal to that of the
voltmeter itself, Captain Cardew uses for the extra circuits
a stretched wire of the same length and section, and
platted under similar conditions as regards cooling as the
wire of the voltmeter itself, both sets of wires being surrounded
with metal tubes, as will be described farther on,
and the tubes, like the metal rods supporting the pulleys
P1, P2, P3, P4, being lamp-blacked on the surface. This extra
wire, which has one end attached to the screw C, passes
over a bone pulley P3 at the top of the instrument, then
down and under the little bone pulley p2, then up and
over the bone pulley P4, and lastly is attached to the
spring S2. The support carrying the pulley p2 is also
attached to a spring S3, hence the stretching of the

second wire which occurs when the current passes
through it is taken up by the contraction of both the
springs S2 and S3, and the wire is kept tight. To prevent
draughts of air cooling the stretched wires they are enclosed
in metal tubes t t, t' t', shown in the figure separated
from the rest of the apparatus. The internal diameter
of these tubes is only a little greater than that of the
circular metal plates, D E, F G, carrying the bearings on
which the pulleys P1, P2, P3 and P4 turn, so that when the
tubes are slipped over the plates and screwed on to J K,
the top of the box, they prevent these plates having any
lateral motion.
To prevent the rods which support the pulleys P1, P2,
P3, P4 expanding and contracting more or less than the
stretched wires when the temperature of the room
changes, which would cause the pointer to move, these
rods may be composed partly of brass and partly of iron,
so that their mean co-efficient of expansion is the same
as that of platinum-silver.
The mechanism contained in the wooden box in the
lower part of the instrument is protected from damage
by the box being closed with a wooden back (not shown
in the figure) which turns on the hinges H H.
The two great advantages of this instrument are :-
First, it has no heating error, since the elevation of the
temperature produced by the passage of the current is
the property of the current made use of; second, it
can be used for measuring alternating P. Ds.
As already stated, when a
current is started round an electro-magnet, it takes a
certain time to reach its maximum value, so that with an
alternating current, which is continually being started in
opposite directions, the effect of the self-induction of the
coil is to practically increase its resistance by an amount
which varies with the rapidity of the alternations ; hence,
apart from the fact that the rapid reversals of magnetism,
which are produced by an alternating current, prevent
an ordinary galvanometer being used for measuring such
a current, even a high resistance dynamometer, which
can be used for measuring an alternating current, cannot be used for measuring an
alternating P. D., for its self-induction would cause it
to practically have a variable resistance, and we have
seen that any variation in the resistance
of a voltmeter varies its sensibility. But as the
self-induction of a straight wire bent back on itself is
very small, the error in Captain Cardew's voltmeter,
arising from self-induction, is negligible, and so this instrument
is much used for measuring an alternating
P. D. It is also dead-beat, direct-reading, not disturbed
by magnets, and fairly portable, although large.
The disadvantages of the instrument, as usually made,
are :- First, it absorbs a good deal of energy ; second, it
cannot be used for measuring a small P. D., for we cannot
make it of thicker wire as we should do in the case
of an ordinary voltmeter intended to measure small
P. Ds., as this would render it sluggish, since a thick
wire traversed by a current heats and cools slowly on
starting and stopping the current; third, there is considerable
vagueness in the readings near the zero point,
and sometimes inaccuracy in the upper parts of the scale.

 

 

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