IMPEDANCE. 



49 



Inductive and Non-inductive Resistance, Strictly speak- 

 ing, no resistance is entirely non-inductive, and no conductor 

 can have inductance without some resistance. It is desirable 

 to form a general idea as to what form of resistance may be 

 considered for practical working purposes non-inductive. 

 Convenient resistances in which the self-induction is entirely 

 negligible at all ordinary frequencies may be made by grouping 

 together incandescent lamps connected in parallel or series- 

 parallel to suit the voltage required. Liquid resistances are 

 also specially suitable for alternating-current work since 

 there is little eating away of the electrodes. A solution of 



Amperes. 



FIG. 22. IMPEDANCE AND RESISTANCE OF WIRE COILS. 

 Direct current. O Alternating current. 



common washing soda works satisfactorily, the strength of 

 the solution being chosen according to the resistance required. 

 The electrodes may be movable iron plates or tubes of any 

 convenient form. 



For general purposes the ordinary resistance frame made of 

 spirals of resistance wire may be considered to be non-inductive, 

 if any of the high-resistance alloys are used. Iron spirals, on 

 the other hand, have considerable self-induction on account 

 of the magnetic nature of the wire, which gives rise to what is 



