A condenser consists of two conducting surfaces separated by an insulator or dielectric. Fig. 53 shows a diagram of a simple condenser in which A and B are two tinfoil sheets separated by a sheet of glass, C.
Fig. 53. Simple Condenser.
If A is connected by means of a wire to a static machine a positive charge will collect on the glass at A and induce a negative charge at B, so that if A and B are connected to a small spark gap the charge will leap the gap in the form of a spark.
When a condenser discharges through a coil of wire, the discharge consists of a large number of exceedingly rapid oscillations or surgings. The first passage of current more than empties the condenser and it becomes charged in the opposite direction, that is, the conducting coatings change their polarity. A reverse discharge then occurs which also oversteps itself and the oscillations thus go on but become rapidly weaker until they die completely. The time consumed in the discharge may have been only a fraction of a second, but during that short period the current perhaps oscillated several thousand times.
If a condenser is discharged through a conductor of high resistance the discharge passes out slowly, and dies away gradually in one direction without oscillating. One of the fundamental equations of wireless telegraphy is therefore that there will be oscillations in a circuit if the resistance in ohms is not greater than the square root of four times the inductance in henries divided by the capacity of the condenser in microfarads.
The capacity or the ability of a condenser to store electricity depends upon the area and form of the conducting surfaces, the thickness of the dielectric between them, and a factor known as the specific inductive capacity of the dielectric. The unit of capacity is called the farad and is defined as the condenser which would be raised to a potential of one volt by a charge of one ampere flowing for one second. A condenser of such a capacity is, because of its enormous size, impractical to construct, and the unit ordinarily used is therefore the microfarad, or one millionth of a farad.
Capacity may be calculated from the following formula:
Capacity equals K(A/D),
where K equals a constant depending upon the specific inductive capacity of the dielectric, A the total area of tinfoil and D the thickness of the dielectric.