EFFECT OF CAPACITY. 99 



must be raised by the flow of electricity from A. The flow 

 will, however, not reach R until the condenser has been 

 charged, as the electricity flows into the condenser first, 

 this being nearer to A. Consequently it is only after the 

 charging current has raised the potential of the condenser 

 plates to the potential of A that the full potential of A can 

 act upon R so as to send current through it. 



In the case of an alternating-current circuit possessing 

 capacity there will be a charging current in the circuit at each 

 reversal of the electromotive force, and the total current 

 flowing will be the sum of the capacity-charging current and 

 the normal current obeying Ohm's law. The charging current 

 will be a quarter of a period in advance of the electromotive 

 force, and hence the resultant current will always have its 

 variations in advance of the voltage by the amount of an 

 angle between zero and 90, depending upon the relative 

 magnitude of the resistance and capacity of the circuit. 



When a condenser is first connected to the terminals of a 

 source of electromotive force, equal and opposite charging cur- 

 rents will flow into the condenser from the two sides. That is, if 

 we consider a positive charge to flow in at the positive terminal, 

 a negative charge will simultaneously flow in at the negative 

 terminal. But an oppositely-directed flow of electricity of 

 opposite sign is equivalent to a direct flow of the same sign. 

 Consequently current may be considered as actually flowing 

 in the same direction in the circuit on both sides of the 

 condenser, and is often said to flow " in " at one terminal 

 and " out " at the other, or to flow through the condenser. 

 As soon as the condenser is charged, the flow must necessarily 

 cease until the potential of the source to which it is connected 

 changes. 



Relation Between Voltage and Current 



The following symbols will be employed : 



Q Quantity of electricity in coulombs. 



K Capacity of condenser in farads. 



E Potential (in volts) of terminal connected to source of 

 higher electromotive force if other terminal is considered 

 to be at zero potential, or E = difference of potential 

 between terminals, if both are connected to sources 

 of electromotive force. 



G Current in amperes flowing into condenser. 



