1 66 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



charge q(= CE) by discharging the condenser through a bal- 

 listic galvanometer. See Chapter X. 



Units of capacity. A condenser is said to have a capacity of 

 one farad when one coulomb of charge is drawn out of one plate 

 and forced into the other plate by an electromotive force of one 

 volt ; C in equation (6 1 ) is expressed in farads when q is 

 expressed in coulombs and E in volts. The farad is an 

 extremely large capacity as compared with capacities ordinarily 

 met with in practice, and the microfarad (one millionth of a farad) 

 is frequently used as a unit. The term abfarad is occasionally 

 used to designate the c.g.s. unit of capacity. A condenser would 

 have one abfarad of capacity if one abcoulomb of charge would 

 be drawn out of one plate and forced into the other plate by an 

 electromotive force of one abvolt. One abfarad is equal to 

 io 9 farads. 



Electric absorption. When a condenser, which has been 

 charged for some time, is discharged and then left standing, a 

 small amount of additional charge collect's on the condenser 

 plates so that a second or third discharge may be taken from the 

 condenser. It seems as if a portion of the initial charge on the 

 condenser were absorbed by the dielectric, this absorbed charge 

 being slowly given back to the condenser plates when these have 

 been discharged. This phenomenon of electric absorption is 

 strictly analogous to the following: A rubber tube which is 

 stretched for some time and then released, comes nearly back to 

 its initial length at once, and then continues to shorten for a long 

 time. If the end of the tube is fixed immediately after the 

 release, the tendency of the tube to continue to shorten will 

 develop a stretched condition in the tube which will show itself 

 by a sudden slight shortening when the tube is released a 

 second time. 



89. Mechanical analogue of the condenser. Figure 103 shows 

 two metal plates AA and BB separated by a dielectric DD 

 and connected to a battery. Figure 104 shows a box separated 

 into two compartments AA and BB by means of a rubber 



