One of the most useful forms of electromagnetic voltmeter is that generally known as a movable coil voltmeter (fig.
This measurement is applicable to the measurement of high potentials, either alternating or continuous, provided that in the case of alternating currents the high resistance employed is wound non-inductively and an electrostatic voltmeter is used.
One much-used electrostatic voltmeter of this type is the Kelvin multicellular vertical pattern voltmeter (fig.
Any form of electrokinetic voltmeter which involves the passage of a current through the wire necessitates the expenditure of energy to maintain this current and therefore involves cost of production.
In the same manner the potentiometer may be used to calibrate a voltmeter by the aid of a divided resistance of known value.
Sumpner in 1891, an electrostatic voltmeter is employed to measure the fall of potential V 1 down any inductive circuit in which it is desired to measure the power absorption, and also the volt-drop V2 down an inductionless resistance R in series with it, and also the volt-drop V3 down the two together.
The voltmeter needle may then be made to record its variations graphically on a drum covered with paper and so to delineate the wave form of the current.
This contact may be made to close the circuit of a suitable voltmeter, or to charge a condenser in connexion with it, and the reading of the voltmeter will therefore not be the average or effective voltage of the alternator, but the instantaneous value of the electromotive force corresponding to that instant during the phase, determined by the position of the rotating contact slip with reference to the poles of the alternator.
A voltmeter is therefore one form of electrometer, but the term is generally employed to describe the instrument which indicates on a scale, not merely in arbitrary units but directly in volts, the potential difference of its terminals.
It is always an advantage, if possible, to employ an electrostatic voltmeter for measuring potential difference if it is necessary to keep the voltmeter permanently connected to the two points.