46 IMPEDANCE. 



For low currents* the impedance is seen to be small. 

 This is owing to the well-known fact that the permeability 

 of iron is low for very low inductions, but rises rapidly as the 

 iron becomes more magnetised, until it reaches a point 

 where it begins to be saturated. The permeability then 

 falls again, but more gradually than it increased at first. 

 These variations are very clearly to be traced in the curve. 

 With a non-magnetic core the impedance would be constant 

 at all currents. 



The curve represents, in fact, the permeability curve for 

 the core of the choking coil, and has the same properties as 

 the permeability curves of iron, although the shape of the 

 curve shown is affected by the fact that the coil had a heavy 

 cast-iron base, which may have slightly affected the slope of 

 the curve. 



The reason for the curve approximating so closely to the 

 shape of a permeability curve is due to the fact that in the 

 impedance */ R 2 + (%irnL}~ the value of R was -15 ohm, 

 and was therefore small compared with the reactance 2 T n L, 

 the lowest value of which was 2-44, rapidly rising to a much 

 higher value. Hence the impedance was nearly the same 

 as 2 IT n.L. Now the value of L is directly propor- 

 tional to the mean permeability of the magnetic path, and 

 consequently the impedance varies in proportion to the 

 permeability. 



The curve which has just been discussed must not be 

 confused with the magnetisation curve, which shows the 

 dependence of the strength of the magnetic field (not 

 permeability) upon the magnetising current. 



* With these low currents a correction had to be made for the voltmeter 

 current which was not negligible compared with the total current. The 

 voltages were read upon an electro-magnetic voltmeter, as the ordinary 

 electrostatic type would not read low enough. In making this correction 

 it was found to be permissible to consider the voltmeter current to 

 be in phase with the voltage at its terminals. Since this was not 

 the case for the current in the coil, the current through the coil had to be 

 determined by subtracting the voltmeter current from the component of the 

 current in the coil which was in phase with the voltage, and determining 

 the resultant of this energy voltage and the idle component of the voltage. 

 The resultant current in the coil determined in this way is the value plotted 

 in the curve. For the larger currents this correction was unnecessary. The 

 lowest reading was obtained by calculation from the coefficient of self- 

 induction (10'5 millihenries) measured on the secohmmeter. 



A simple method of avoiding the correction for the voltmeter current 

 would have been to employ a key in series with the voltmeter, which would 

 be closed' when the voltage is read and opened when the current is read. This 

 would only be permissible when the voltmeter resistance is high compared 

 with the resistances in the circuit. 



