Page:Popular Science Monthly Volume 92.djvu/645

 Electrical Devices and How They Work

IV. — Theory and Fundamental Principle of the Condenser

A condenser consists of a material that has the property of holding a charge of electricity

By Peter J. M. Clute, B. E.

��IF a metal plate A is mounted on an in- sulated stand and connected with an electroscope, as in Fig. 1, and if an- other plate B, similarly mounted, is con- nected to the ground, then, when an elec- trostatic charge is placed on A, it will be

���fig 1 Ground— ^=-

A diagram illustrating the action of an electrostatic charge between two plates

observed that the leaves of the electro- scope collapse as B is brought close to A and diverge farther as B is moved away from A. This merely shows that the po- tential of A is lessened when the distance between the plates is diminished, in spite of the fact that the quantity of electricity on A has remained unchanged. If addi- tional charges of positive sign are con- veyed to A, it vv'ill be found that many times the original amount of electricity may be placed on it when B is in proxim- ity to it, before the body regains its origi- nal potential.

It can be stated, therefore, that the capacity of plate A for retaining electric- ity is very greatly increased by bringing near it another conductor connected to earth. It is evident from this statement that the capacity of a body is measured by the amount of electricity that can be put upon it in order to raise its potential to a certain point. Under these circum- stances the charge on plate A is said to be bound by the opposite electricity on plate B.

An arrangement of this sort consisting of two conductors separated by a non- conductor or dielectric, as air, mica, rub- ber, or paraffined paper, is called a con-

��denser. When the conducting points are very close together and one of them is grounded, the capacity of the system may be thousands of times as much as that of a single plate. If these two plates are connected to the two terminals of a bat- tery, or other source of electrical energy, as shown in Fig. 2, they acquire equal and opposite charges of electricity, corre- sponding in sign to the terminals to which they are connected. The final constant difference in potential between the two plates is that of the two terminals; or, what is the same thing, each plate assumes the potential of the point to which it is connected. If the leads to the condenser are of negligible resistance, this difference is produced almost instantaneously, that is, the condenser charges in a negligible time.

The flow of current will be at its maxi- mum at the instant the E. M. F. is ap- plied, but will rapidly fall off, so that in a fraction of a second, the current will prac- tically have ceased flowing, and the con- denser will be charged. This condition will exist as long as the condenser is con- nected to the battery, or other electrical source. The condenser acts as if it had

��Condenser baffertj

— ^ ;]lil|l|8!l|l|llh = —

rig. 2

Two conductors separated by a non-con- ductor within a circuit makes a condenser

��acquired a counter E. M. F, as it becomes charged, thus bringing into play a ten- dency to retard the flow of current. This counter effect increases as the con- denser is charged, until it becomes equal and opposite to the E. M. F. of the bat- tery. If the battery be disconnected and

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