98 EFFECT OF CAPACITY. 



(i.e., a current) between the machine and both poles of the 

 condenser. 



The following general explanation may help to make the 

 succeeding more precise reasoning understood by those not 

 already familiar with the nature of capacities. 



Effect of Capacity in the Circuit: General Explanation. 

 If a constant electromotive force be applied to a circuit 

 possessing neither self-induction nor capacity, a current is 

 instantly produced the value of which is given by Ohm's 

 law: 



n Electromotive force 



Current = - . 



Resistance 



If, however, the circuit possesses electrostatic capacity as 

 well as resistance, a charging current will have to flow into 

 the circuit before the voltage acting upon the resistance rises 

 to the full value of the applied electromotive force. Conse- 

 quently, at the first moment of application of the voltage a 

 current will flow into the circuit, and the strength of this 

 current will depend mainly upon the capacity of the circuit, 

 and only partly upon its resistance. It is only after this 

 initial current has charged the circuit that the normal value 

 of the current will be established, and the current strength 

 will be that given by Ohm's law. Thus in a circuit containing 

 a condenser or having electrostatic capacity there will be an 

 initial rush of current which may be considered as flowing 

 in advance of the normal current of the circuit. 



This may be illustrated by the diagram Fig. 43. Suppose 



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R 



FIG. 43. CIRCUIT POSSESSING CAPACITY AND RESISTANCE. 



that C represents a condenser with one pole connected to the 

 circuit, and the other pole earthed. A is a point in the circuit 

 which is suddenly connected to a source of high potential. 

 This will cause a flow of current from A to B. Before the 

 high potential applied at. A can produce a current through 

 the resistance R, the potential at the end of R nearest to A 



