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.
Volts, Lost.
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.
Voltmeter, Electrostatic.
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.
Vulcanite.
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.