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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