nS SINGLE-PHASE CURRENTS. [Exp. 



and A r 2 , the results being dependent upon three voltmeter read- 

 ings and current, and not dependent upon the wattmeter (as in 

 the wattmeter method, 46), nor upon the measurement of resist- 

 ance (as in the impedance method, 47). 

 Referring to Fig. 9, we have 



hence 



X 2 = L 2 d) = C A. r- /, 

 and 



Z* 2 = X 2 r~ ft) = A 2 ~ r~ 



In applying the three-voltmeter method, greatest accuracy is 

 obtained when E t E 2 . If an electrostatic voltmeter is used, no 

 error is introduced on account of power consumed in the instru- 

 ment, 



50. Three-voltmeter Method for Measuring Power. Before the 

 perfection and general introduction of the wattmeter, the three-volt- 

 meter method for measuring power was used ; this is now obsolete for 

 practical testing. The procedure was as follows: 



Given a device RJL a (which might be, for example, a transformer) 

 the power in which is to be measured. Connect in series a non- 

 inductive resistance R lt as in Fig. 8, and read E, E v 2 and /. The 

 power in RyL^ (see Fig. 9) is 



See Bedell and Crehore's 'Alternating Currents, p. 232. The weak 

 point in the method is that small errors in observation make large 

 errors in the result. The three-ammeter method, with a non-inductive 

 resistance in parallel with the apparatus under test as in Fig. 10, is 

 open to the same objection. 



51. (d) Resistance and Coil in Parallel. In parallel cir- 

 cuits,* currents combine vectorially, the main current being the 

 'vector sum of the branch currents. 



* (5ia). Currents are proportional to admittances; hence admittances 

 may be added as vectors. The admittance of several circuits in parallel 



