Page:Popular Science Monthly Volume 92.djvu/646

 630

��Popular Science Monthly

��the terminals of the condenser connected together, as in Fig. 3, the charge will flow out and will result in a current of short duration. This current is at its maximum when the terminals are first connected, but it soon dies down to zero value.

When a condenser is charged, the po- tential difference at the terminals does not instantly come to a maximum value; in other words, a certain time elapses before the condenser reaches full charge. This apparent absorption is due to an action on the dielectric surface. At discharge, a certain time also passes before the pre- vious charge is entirely removed; some of the charge has been absorbed into the die- lectric, which charge is called residual. A condenser exhibiting this quality pos- sesses residual absorption. Hence, the actual capacity of a given condenser is not definite, depending as it does upon the

��Condenser

��rig. 5

With the battery removed and the circuit made complete the chaige soon flows out

amount of residual absorption and leakage.

Condensers may be connected in par- allel, as in Fig. 4, or in series, as shown in Fig. 5. The combined capacity of two condensers in parallel is equal to their sum. If Ci and C2 are the capacities of the two condensers illustrated diagram- matically in Fig. 4, their combined capa- city will equal C, -f C... This is true for any number of condensers connected in parallel; hence, if a number of condensers are connected in parallel, their combined capacity is equal to the sum of all the capacities.

The combined capacity of two con- densers in series is equal to unity divided by the sum of the reciprocals of the two capacities; or, referring to P^'ig. 5:

��C =

��c,

��1

��c, + c.

��C, Co

This rule applies to any number of con- densers in series.

��Condensers are made by taking a large number of tinfoil sheets and separating them by alternate sheets of paraffined paper, mica, or other insulating material. The whole mass is pressed tightly to-

��ol.

��Tip

��C,

�� ��D—

��fig. 4

With condensers connected in parallel their combined capacity is equal to their sum

gether, one set of sheets being connected with one terminal and the alternate set with the other, as illustrated in Fig. 6. It should especially be noted that no electrical connection exists between the sets of plates connected to the two termin- als, since it is this property of inductivity of the dielectric that enables the con- denser to store up such an enormous charge of electrical energy.

The quantity of electricity held by the condenser may be made greater by in- creasing the charging E. M. F. and is directly proportional to this E. M. F. In addition, it is found that for a given voltage, the quantity of electricity which the plates will acquire depends upon their size, their separation, and the dielectric or insulation between them. The quantity of electricity held by either plate of a cliarged condenser, represented by Q, may be written equal to the product EC,

oTi TtO

��fig. 5

Also if the condensers are connected in series their combined value equals their sum

where E is the charging E. M. F. and C is a constant factor which takes into account the construction of the condenser. This factor C is known as the capacity of the condenser.

Thus, we may write, C = Q E, or the capacity of a condenser is the quantity of

�� �