4 -A] SERIES AND PARALLEL CIRCUITS. 



PART I. ELEMENTARY PRINCIPLES. 



2. Defining Relations. In a direct current circuit, the cur- 

 rent which flows is I = E-^-R, irrespective of whether the cir- 

 cuit is inductive or not; the power expended is the product of 

 electromotive force and current. 



3. In a non-inductive* alternating current circuit, this is also 

 true; the current is determined by the resistance, as in a direct 

 current circuit, and the power is the product of electromotive 

 force and current ; thus, 



I = E-t-R; W = EL 



The impedance, defined below, consists in this case of the 

 resistance R only. 



4. In an inductive alternating current circuit, the current is 

 less than E^-R and the power is less than El; thus, 



I = E-^-Z; W = EIX power factor. 



The impedance Z, defined as the volts per ampere, is greater 

 than the resistance R on account of the reactance X ; thus, 



The reactance (defined in 40) for an inductive circuit has a 

 value X = Lw, where L is the inductance, or coefficient of self- 

 induction of the circuit, and w is 2?r X frequency in cycles per 

 second ( I, Exp. 3~A). Impedance and reactance are expressed 

 in ohms. It is seen that inductive reactancef depends not only 



* (3a). A circuit is inductive when a current in it sets up a magnetic 

 field ( 14) ; it is non-inductive when a current in it produces no magnetic 

 field. A circuit is never entirely non-inductive, but may be made nearly 

 so. This is practically accomplished when the outgoing and return con- 

 ductors are placed so close together that the magnetic effects of the 

 currents in the two conductors neutralize each other. In a solenoid this 

 is accomplished by using a double winding, the currents in the two halves 

 of which flow in opposite directions. 



f (4a). In a circuit with capacity C, the reactance is i/Cw. When L 

 and C are both present, the total reactance is the difference between the 

 capacity reactance and inductive reactance; X = Lu i/Cw. See 57. 



