IMPEDANCE. 37 



M l} M.,. Source of alternating current. 



A. Ammeter for reading current in circuit. 



Vi. Voltmeter, reading voltage across impedance. 



F 2 . Voltmeter, reading voltage across non-inductive 



portion of impedance. 

 R. Variable resistance. 

 RI. Non-inductive variable resistance forming part 



of impedance to be measured. 

 r. Inductive coil. 

 S. Switch. 



Instructions. Connect in series two variable non-inductive 

 resistances, the inductive coil and an ammeter. One variable 

 non-inductive resistance R^ and the coil r together make the 

 impedance to be measured. Connect voltmeters so as to 

 measure the volts across the non-inductive resistance R l and 

 across the impedance R l + r respectively. 



A single i jsistance may be used instead of the two R, R l 

 shown in the diagram. This is then kept constant throughout 

 the experiment, and the point of contact of the voltmeter 

 is moved along it between each reading, thus changing the 

 amount of the resistance -included in the impedance 

 measured, while leaving the total resistance in the circuit 

 constant. 



Supply the circuit thus formed with alternating current 

 of constant periodicity. 



By varying R and R l together alter the value of _R t while 

 keeping the sum of R and R l about constant, so as to maintain 

 the current at an approximately constant value, and take 

 readings on the instruments for a series of values of R^. 



If the resistance of the coil r is not known make a measure- 

 ment of this by sending a measured direct current through 

 it, and measuring the drop of volts across its terminals. 



Its resistance is then given by resistance = - 



current. 



For each series of readings taken with the alternating 

 current the value of the impedance should be calculated 



thus : Impedance = -^ . Also the value of the non-inductive 

 



part R! of the impedance should be calculated R l = -=?' the 



G 



resistance of the coil r (which if not known must be measured 

 by direct current as stated above) should be added to the 

 value of RI. 



