126 ADVANCED ELECTRICITY AND MAGNETISM. 



plate and forced into the other plate of a condenser when an 

 electromotive force of E volts is connected so as to act upon the 

 condenser, and C is a constant for a given condenser. The 

 factor C is adopted as a measure of what is called the capacity 

 of the condenser. Therefore, a condenser would have unit 

 capacity if an electromotive force of one volt would draw one 

 coulomb of charge out of one plate and force one coulomb of 

 charge into the other plate of the condenser. 



It is evident from the above equation that C, the capacity 

 of a condenser, is expressed in coulombs-per-volt. One coulomb- 

 per-volt is called a. farad, that is to say, a condenser has a capacity 

 of one farad when an electromotive force of one volt will draw one 

 coulomb out of one plate of the condenser and force one coulomb 

 into the other plate of the condenser. 



Condenser capacities as usually encountered in practice are 

 very small fractions of a farad. Thus the capacity of a condenser 

 made by coating with tin foil the inside and outside of an ordi- 

 nary one-gallon glass jar would be about one five-hundred- 

 millionth of a farad, or 0.002 of a microfarad. A microfarad is a 

 millionth of a farad, and in practice capacities of condensers 

 are usually expressed in microfarads. 



The approximate dimensions 

 of a one-microfarad condenser 

 are as follows : 501 sheets of tin 

 foil separated by sheets of para- 

 ffined paper 0.02 inch in thick- 

 ness, the overlapping portions 

 of the sheets of tin foil being 10 

 inches by 10 inches, as shown in 

 Fig. 84. 



Fig. 84. Two pieces of metal of any 



shape separated by insulating 



material constitute a condenser; the only reason for using sheets 

 of metal with thin layers of insulating material between is to 

 obtain a large capacity in a small space. 



