IMPEDANCE. 51 



Self-induction measured by secohmmeter = 2-3 millihenries. 

 Ohmic resistance = 20-66 ohms. 

 Reactance = 2 TT n L = -604 when n 42. 

 .-. Impedance - -x/20'66 2 + "604 2 = 



= 20-664 apparent ohms. 



Hence the error introduced in neglecting self-induction 

 would only affect the impedance to the extent of about one- 

 quarter per 1,000, when used for such small currents. 



With a non-magnetic material the self-induction would be 

 the same for all current densities, and the resistance might 

 safely be considered non-inductive .for any current of this 

 frequency. 



The important influence of the magnetic material of which 

 the resistance in this case is composed is plainly shown by the 

 wide and increasing divergence between direct and alternating 

 current readings, as the current density increases, showing 

 that iron resistances cannot be considered as non-inductive 

 with larger currents, although a similar resistance of non- 

 magnetic material may generally be considered to be so. 



The figures given above form a useful rough basis for 

 estimating the self-induction of any resistance spiral, as the 

 self-induction varies directly as the square of the number 

 turns in the spiral, directly as the sectional area of the coil, 

 and inversely as the length. 



t A 

 Thus self-induction oc -= 



V 



Impedance of Armature of an Alternator. A practical 

 example of a circuit in which the impedance is affected by all 

 the factors determined separately in the foregoing experiments 

 is the core of the armature of an alternator or synchronous 

 motor. 



The self-induction of the armature will depend on the exact 

 position of the armature relative to the poles, since the magnetic 

 circuit of the armature is completed through the poles or the 

 pole faces. The extent to which the position affects the 

 self-induction will depend on the nature of the winding and 

 form of the magnetic system of the machine. 



A distributed winding will show a small variation, since 

 some of the conductors will always be opposite to part of the 

 magnets, while the maximum value of its self-induction will 

 not be high. 



