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.
In actual practice the receiving instrument is so sensitive that the difference of potential between the two coatings of the condenser C3 produced by the incoming signal is only a very small fraction of the potential of the battery B.
If the electrified body is touched against the upper plate whilst at the same time the lower plate is put to earth, the condenser formed of the two plates and the film of air or varnish becomes charged with positive electricity on the one plate and negative on the other.
As an auxiliary to air cooling the stack may be cooled by a slow stream of water trickling down the outside of the pipes, or, in certain cases, cold water may be injected into the condenser in the form of a spray, w here it meets the ascending vapours.
Across the arc is a transverse or radial magnetic field, and the electrodes are connected by an oscillatory circuit consisting of a condenser and inductance.
The condenser commonly used is an old retort.
On the other hand, if a closed oscillation circuit is constructed having capacity and considerable inductance, then oscillations can be set up in it by very small periodic electromotive forces provided these have a frequency exactly agreeing with that of the condenser circuit.
The impedance of the primary or alternator circuit is so adjusted that when both the chokers are in circuit the current flowing is not sufficient to charge the condensers; but when one choker is short-circuited the impedance is reduced so that the condenser is charged, but the alternating arc is not formed.
For the purpose of a standard condenser a number of concentric metal tubes may be arranged on an insulating stand, alternate tubes being connected together.
On insulating the lower plate and raising the upper plate by the glass handle, the capacity of the condenser formed by the plates is vastly decreased, but since the charge on the lower plate including the gold leaves attached to it remains the same, as the capacity of the system is reduced the potential is raised and therefore the gold leaves diverge widely.