Ammeters to measure the volume, and voltmeters to determine the pressure of current supplied to the baths, should also be provided.
There are methods of measuring electrical power by means of electrostatic voltmeters, or of quadrant electrometers adapted for the purpose, which when so employed may be called electrostatic wattmeters.
Voltmeters may be divided into two classes, (a) electrostatic, (b) electrokinetic.
Electrostatic voltmeters are based on the principle that when two conductors are at different potentials they attract one another with a force which varies as the square of the potential difference (P. D.) between them.
In the case of high tension voltmeters, the movable plate takes the form of a single plate of paddle shape, and for extra high tensions it may simply be suspended from the end of a balanced arm; or the movable system may take the form of a cylinder which is suspended within, but not touching, another fixed cylinder, the relative position being such that the electric forces draw the suspended cylinder more into the fixed one.
Another class of voltmeters comprises the electrokinetic voltmeters.
In any case of potential difference measurement it is essential not to disturb the potential difference being measured; hence it follows that in electrokinetic voltmeters the wire connecting the two points of which the potential difference is to be measured must be of very high resistance.
Electromagnetic voltmeters may therefore be thermal, electromagnetic or electrodynamic. As a rule, electromagnetic voltmeters are only suitable for the measurement of relatively small potentials - o to 200 or 300 volts.
Hot wire voltmeters, like electrostatic voltmeters, are suitable for use with alternating currents of any frequency as well as with continuous currents, since their indications depend upon the heating power of the current, which is proportional to the square of the current and therefore to the square of the difference of potential between the terminals.
Electromagnetic voltmeters consist of a coil of fine wire connected to the terminals of the instrument, and the current produced in that wire by a difference of potential between the terminals creates a magnetic field proportional at any point to the strength of the current.
Like the corresponding ammeters, they have the great advantage that the scales are equidivisional and that there is no dead part in the scale, whereas both the electrostatic and electrothermal voltmeters, above described, labour under the disadvantage that the scale divisions are not equal but increase with rise of voltages, hence there is generally a portion of the scale near the zero point where the divisions are so close as to be useless for reading purposes and are therefore omitted.
(iii.) The instrument should have no temperature correction; this is a good quality of electrostatic instruments, but in all voltmeters of the electrokinetic type which are wound with copper wire an increase of one degree centigrade in the average temperature of that wire alters the resistance by 0.4%, and therefore to the same extent alters the correctness of the indications.
This last point is important in connexion with voltmeters used on the switchboards of electric generating stations, where relatively strong electric or magnetic fields may be present, due to strong currents passing through conductors near or on the board.
Electrostatic voltmeters are also liable to have their indications disturbed by electrification of the glass cover of the instrument; this can be avoided by varnishing the glass with a semi-conducting varnish so as to prevent the location of electrostatic charges on the glass.
Selfregistering voltmeters indicate at any moment the potential difference in every tank, and therefore give notice of short circuits occurring at any part of the installation.
Electrostatic voltmeters are now almost entirely used for the measurement of high voltages from 2000 to 50,000 volts employed in electrotechnics.