The importance of this investigation resides in the fact that an electrometer of the above pattern can be used as a wattmeter, provided that the deflection of the needle is proportional to the potential difference of the quadrants.
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.
If these spark balls are set at the right distance, then when the potential difference accumulates the antenna will be charged and at some stage suddenly discharged by the discharge leaping across the spark gap. This was Marconi's original method, and the plan is still used under the name of the direct method of excitation or the plain antenna.
The energy stored up in the jar in joules is expressed by the value of CV 2, where C is the capacity measured in farads and V the potential difference of the coatings in volts.
When this is the case the amplitude of the potential difference of the surfaces of the tubular condenser becomes a maximum, and this is indicated by connecting a vacuum tube filled with neon to the surfaces of the condenser.
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.
In a small piece of soft iron, as in the case of the corresponding ammeters, and this in turn may be made to displace an indicating needle over a scale so that corresponding to every given potential difference between the terminals of the instrument there is a corresponding fixed position of the needle on the scale.
In this case a highresistance wire is connected between the points of which the potential difference is required, and from some known fraction of this resistance wires are brought to an electrostatic voltmeter, or to a movable coil electromagnetic voltmeter, according as the voltage to be measured is alternating or continuous.
The principle on which the instrument works is as follows: Suppose any circuit, such as an electric motor, lamp or transformer, is receiving electric current; then the power given to that circuit reckoned in watts is measured by the product of the current flowing through the circuit in amperes and the potential difference of the ends of that circuit in volts, multiplied by a certain factor called the power factor in those cases in which the circuit is inductive and the current alternating.
Necessarily obey a law of deflection making the deflections proportional to the potential difference of the quadrants, but that an electrometer can be constructed which does fulfil the above law.