SINGLE-PHASE CURRENTS. [Exp. 



PART II. MEASUREMENTS. 



21. The following tests require a resistance, which is non- 

 inductive and is designated R l ; and a coil, which is inductive and 

 is designated R 2 L 2 . It is desirable to have the resistance and the 

 coil take currents which are comparable in value with each other, 

 for the frequency at which the tests are made; thus, if at no 

 volts, 60 cycles, the coil takes a current of 10 amperes, the resist- 

 ance should be so selected that at no volts it takes a current of, 

 say, from 5 to 20 amperes. Except for 28, the coil should not 

 have an iron core, so that there are no losses except RI 2 . 



For the tests of 26a (which may precede the main tests), the 

 windings of the coil should be divided in two equal parts, which 

 can be connected in series and in parallel. 



22. The instruments required consist of a voltmeter, capable 

 of reading the supply voltage and lower voltages; an ammeter 

 capable of measuring the combined currents of the coil and resist- 

 ance ; and a wattmeter having a voltage range corresponding to 

 the range of the voltmeter and a current range corresponding to 

 the range of the ammeter. 



A voltmeter switch will be found convenient for the series tests 

 (29-31) and an ammeter switch for the parallel tests (32-34). 



On all tests the frequency should be known. 



23. (a) Resistance Alone. With an adjusting resistance in 

 series, as in Fig. 4, connect the resistance R : to the supply circuit 

 (say no volts, 60 cycles) and measure the current /, the voltage 

 E at the terminals of R lf and the watts W consumed by R lm The 

 current coil of the wattmeter is connected in series as an ammeter 

 and the potential coil in shunt as a voltmeter, the arrangement* 

 of instruments being shown in Fig. I, Exp. 5-8. 



* (233). In these tests no account is ordinarily to be taken of the fact 

 that the instruments themselves consume a certain small amount of 

 power, as fully discussed in Appendix III., Exp. 5-A ; this fact, however, 

 should not be neglected in accurate testing, as for example in the accurate 

 determination of L by the impedance method, 47. 



