Electrical Accumulators or Condensers

Consider an insulated conductor in the form of a plate, which is connected with one pole of a battery; let the other pole, suppose for clearness the negative one, be put to earth, it will be at zero potential. The plate will have a charge of positive electricity on it depending on its form, and its potential will be equal to the E.M.F. of the battery.

Take another plate, connected with the earth, and bring it into the neighbourhood of the first plate. This second plate will be at potential zero, and its presence will tend to lower the potential of the first plate, and thus will produce a flow of positive electricity from the battery to the first plate, sufficient to raise its potential again to- that of the positive pole of the battery. The quantity of electricity which thus flows in will depend on the form and relative position of the two plates, and the nature of the insulating medium which separates them. The flow of electricity will last but an exceedingly short time; and, if allowed to pass through a ballistic galvanometer, will produce a sudden throw of the needle of the nature described on p. 466. If ft be the angle through which the needle is deflected, then, as we have seen, the quantity of electricity which passes is proportional to sin(0.5β).

It is not necessary to connect the negative pole of the battery and the second plate of the condenser to earth; it will be sufficient if they be in electrical communication with each other; in either case the difference of potential between the plates will be equal to the E.M.F. of the battery.

Neither is it necessary that the two plates of the condenser should be capable of being separated; the effects will be exactly the same if we suppose one plate to be in connection with the negative pole of the battery, and then make contact by means of a key between the second plate and the positive pole. The condenser can be discharged by putting its two plates in metallic connection by means of a wire.

Moreover it can be shown that if there be a quantity Q of positive electricity on the one plate of the condenser, there will be a quantity -Q on the other. (See Maxwell's 'Elementary Electricity,' p. 72.) By the charge of the condenser is meant the quantity of electricity on the positive plate.