Vulcanized india rubber which by high proportion of sulphur and proper
vulcanization has been made hard. It is sometimes distinguished from
ebonite as being comparatively light in color, often a dull red, while
ebonite is black. For its electrical properties see Ebonite.

Both substances have their defects, in producing surface leakage.
Washing with weak ammonia, or with dilute soda solution, followed by
distilled water, is recommended for the surface, if there is any trouble
with surface leakage. It may also be rubbed over with melted paraffine
wax.

A voltmeter based on the lines of the quadrant electrometer. It includes
two sets of quadrants, each oppositely excited by one of the two parts,
whose potential difference is to be determined. They attract each other
against a controlling force as of gravity.

One form has the two sets poised on horizontal axes, bringing the parts
so that the flat quadrants move in vertical planes.

In another form a number of quadrants are used in each set, the members
of the two sets alternating with each other. One set is fixed, the
others move and carry the index.

The volts at the terminals of a dynamo at full load fall short of their
value on open circuit. The difference of the two values are termed lost
volts.

A voltmeter in which the current passing through its conductor heats
such conductor, causing it to expand. Its expansion is caused to move an
index needle. By calibration the movements of the needle are made to
correspond to the potential differences producing the actuating currents
through it. The magnetic action of the current plays no part in its
operation. It is the invention of Capt. Cardew, R. E.

The construction of the instrument in one of its most recent forms is
shown in the cut. On each side of the drum-like case of the instrument
are the binding screws. These connect with the blocks m and n. To these
the fine wire conductor is connected and is carried down and up over the
two pulleys seen at the lowest extremity, its centre being attached to
c. From c a wire is carried to the drum p, shown on an enlarged scale on
the left of the cut. A second wire from the same drum or pulley connects
to the spring S. The winding of the two wires is shown in the separate
figure of c, where it is seen that they are screwed fast to the
periphery of the little drum, and are virtually continuations of each
other. By the screw A the tension of the spring S is adjusted.

On the shaft of the little drum p is a pinion, which works into the
teeth of the cog-wheel r. The shaft of r is extended through the dial
of the instrument, and carries an index. The dial is marked off for
volts; g g and h h are standards for carrying the pulleys.

The action of the instrument is as follows. The current passing through
the wire heats it. This current by Ohm’s law is proportional to the
electro-motive force between the terminals. As it is heated it expands
and as it cools contracts, definite expanding and contracting
corresponding to definite potential differences. As the wire expands and
contracts the block or pin c moves back and forth, thus turning the drum
p and cogwheel r one way or permitting it to turn the other way under
the pull of the spring S.

In this construction for a given expansion of the wire the piece c only
moves one half as much. The advantage of using a wire twice as long as
would be required for the same degree of movement were the full
expansion utilized is that a very thin wire can be employed. Such a wire
heats and cools more readily, and hence the instrument reaches its
reading more quickly or is more deadbeat, if we borrow a phraseology
properly applicable only to instruments with oscillating indexes.

In the most recent instruments about thirteen feet of wire .0025 inch in
diameter, and made of platinum-silver alloy is used.

If the potential difference to be measured lies between 30 and 120 volts
the wire as described suffices. But to extend the range of the
instrument a resistance in series is required. If such resistance is
double that of the instrument wire, and remains double whether the
latter is hot or cold the readings on the scale will correspond to
exactly twice the number of volts. This is brought about in some
instruments by the introduction in series of a duplicate wire, precisely
similar to the other wire, and like it, carried around pulleys and kept
stretched by a spring.

[Transcriber's note: If the series resistance is twice that of the
voltmeter, the indicated voltage will be ONE THIRD of the total
voltage.]

Thus whatever ratio of resistance exists between the two wires cold, it
is always the same at any temperature, as they both increase in
temperature at exactly the same rate. Tubes are provided to enclose the
stretched wires and pulleys, which tubes are blackened.

The voltmeter is unaffected by magnetic fields, and, as its
self-induction is very slight, it is much used for alternating currents.
The tubes containing the wire may be three feet long.

Its disadvantages are thus summarized by Ayrton. It absorbs a good deal
of energy; it cannot be constructed for small potential differences, as
the wire cannot be made thicker, as it would make it more sluggish;
there is vagueness in the readings near the zero point and sometimes
inaccuracy in the upper part of the scale.

A voltmeter for use in running batteries. In one form (Wirt’s) it is
constructed for a low range of voltage, reading up to two and a half
volts and having exactly one ohm resistance, thus giving the battery
some work to do.

An instrument for determining the potential difference of any two
points.

In many cases it is a calibrated galvanometer wound with a coil of high
resistance. The object to be attained is that it shall receive only an
insignificant portion of current and that such portion shall suffice to
actuate it. If connected in parallel with any portion of a circuit, it
should not noticeably diminish its resistance.

The divisions into which ammeters range themselves answer for
voltmeters. In practice the same construction is adopted for both. The
different definitions of ammeters in disclosing the general lines of
these instruments are in general applicable to voltmeters, except that
the wire winding of the coils must be of thin wire of great length. The
definitions of ammeters may be consulted with the above understanding
for voltmeters.

In the use made of voltmeters there is a distinction from ammeters. An
ammeter is a current measurer and all the current measured must be
passed through it. But while a voltmeter is in fact a current measurer,
it is so graduated and so used that it gives in its readings the
difference of potential existing between two places on a circuit, and
while measuring the current passing through its own coils, it is by
calibration made to give not the current intensity, but the
electro-motive force producing such current.

In use it may be connected to two terminals of an open circuit, when as
it only permits an inconsiderable current to pass, it indicates the
potential difference existing between such points on open circuit. Or it
may be connected to any two parts of a closed circuit. Owing to its high
resistance, although it is in parallel with the intervening portion of
the circuit, as it is often connected in practice, it is without any
appreciable effect upon the current. It will then indicate the potential
difference existing between the two points.

The legal volt based upon the legal ohm. It is equal to 1.00112 B. A.
volt.

A form of easily read voltameter for use in electric light stations and
for similar work.

The unit of electric work; the watt-second; it is equivalent to
1.0E7     ergs.
.24068   gram degree C. (calorie)
.737337  foot lbs.,
.00134   horse power seconds.

The volt based upon the congress or legal ohm; the legal volt.