44 



IMPEDANCE. 



readings obtained in this experiment if the resistance of 

 the coil and frequency of the current are noted. This is 

 sometimes important in the case of rotating machinery, as 

 the self-induction affects the wave form of the voltage 

 generated. A more obvious use of the results is to show 

 the range of application of a choking coil required to work 

 on arc lamp circuits of various voltages. 



\ 



u 



1 



a 



2 3 4 b 6 



Inches of Air Gap. 

 FIG. 20. DEPENDENCE OF IMPEDANCE ON AIR GAP. 



EXPERIMENT VI. DETERMINATION OF DEPENDENCE OF 

 IMPEDANCE UPON SELF-INDUCTION. 



(2) Due to alteration in strength of current. 



General. The strength of the magnetic field produced in 

 an iron core by a given current depends upon the permeability 

 of the iron. 



Since the self-induction of a coil having a magnetic circuit 

 partly composed of iron is proportional to the strength of 

 field produced by each ampere of current flowing in the coil, 

 any variation in the permeability of the iron will change the 

 coefficient of self-induction. The permeability of iron changes 



