I2 4 SINGLE-PHASE CURRENTS. [Exp. 



4. Data. Let the circuit be as shown in Fig. 8, Exp. 4~A, 

 in which R^ is a non-inductive resistance and R 2 L 2 is a coil (with- 

 out iron) with resistance R 2 , inductance L 2 and reactance X 2 . 

 The impressed electromotive force E should be constant; in case 

 E varies, all readings should be reduced by direct proportion to 

 correspond to some constant value of E; an adjusting resist- 

 ance, as shown in Fig. 8, Exp. 4~A, is unnecessary. The fre- 

 quency should be constant. 



With an ammeter, measure the current 7. With a voltmeter, 

 measure the various falls of potential as follows: E, the im- 

 pressed electromotive force; E lt the fall of potential around 

 the non-inductive resistance R^; E 2) the fall around the coil 



The error due to the current taken by the voltmeter, although 

 negligible for a circuit in which the current is large, becomes 

 appreciable when the current is small ; this error may be avoided 

 by using an electrostatic voltmeter, which takes only sufficient 

 current to charge the instrument. 



5. Take a series of readings for decreasing values of R^ 

 throughout the range that it is possible to read x and E 2 . 



6. Repeat at a second frequency. 



7. Repeat at one frequency with an iron core in the coil. 



8. Measure the resistance of the coil, R 2) by direct current, 

 fall-of-potential method, 17, Exp. i-A. 



9. Results. For one set of readings, draw a triangle, OAB, 

 Fig. i, with the observed values of E, E^ and E 2 as the three sides. 

 Lay off OD in the direction of OB, equal to the current I, in any 

 convenient scale. Produce OB to C by an amount B,C = RJ, the 

 electromotive force to overcome the resistance and supply the 

 RI 2 losses in the coil. OC is the electromotive force to over- 

 come the resistance of the entire circuit. The current and elec- 

 tromotive forces are now represented in magnitude and direction 

 for one value of the resistance. Fig. I is the typical diagram for 



