(KX EI)±2.712, where K is the capacity of, the charged condenser in farads, and E is the charged electromotive force in volts. It can be shown that the charge of a condenser rests on opposite sides of the dielectric, and that in charging the condenser as much electricity leaves plate as enters plate A. The capacity of a condenser varies with the distance between its opposite plates, being greater the nearer they are together, and increases with the surface of the plates. The capacity also varies with the insulating material or dielectric used to sepa rate or insulate the plates. The property of di electrics to which this so-called inductive effect is due is termed specific inductive capacity. The property which this inductive capacity seem ingly imparts to conductors is termed electro static capacity, or "capacity.° The inductive capacity of air is taken as the standard. Air being unity, the specific inductive capacity of paraffin is about 2; vulcanized India-rubber, 2.94; gutta-percha, 4.5; mica, 5; flint glass, 6.5 to 10. The Leyden jar is a well-known type of condenser. The most common form of con denser is generally constructed of many sheets of tinfoil, separated by thin sheets of insulating material, such as paraffin paper, mica or glass; the alternate sheets of tinfoil are connected to gether metallically at their ends as indicated in Fig. 2.
In malcing a condenser for radiography, the glass plate type is recommended as inexpensive and durable, and also much lighter than oil immersed types. Photographer's negative glass, tinfoil and shellac are the materials. The foil should be cut to the required size (6 X 8 inches is convenient), and carefully smoothed to take out all wrinkles. The glass should be cleaned with alcohol and coated with shellac, then covered with the foil, and rolled or "squeegeed" so as to be perfectly smooth. In assembling the plates lugs should be placed between them. A unit may be made of 10 plates which are bound together with wire or suitable tape, and immersed for one hour in a bath of equal parts of hot melted rosin and beeswax, then allowed to drain and dry. This gives a con denser unit thoroughly moisture proof, with a capacity of .01 microfarad, which is suitable for the ordinary half-kilowatt wireless trans former for the standard 200-metre wave-length. It is good practice to make four such units, placing two multiple sets of two in each series; this reduces the strain on the condenseri, with out altering the capacity. They may be mounted in substantial open-side wooden boxes to protect the plates from injury.