

5-A1 STUDY AND OPERATION. H9 



coil of the wattmeter.. This error can be neglected, when the load 

 current is large. The ammeter reading would be correct if the 

 potential coil of the wattmeter (and voltmeter, if one is used) were 

 opened when the ammeter is read; sometimes this is allowable under 

 steady conditions, but simultaneous readings of all instruments are 

 usually more accurate. 



44. Combinations of Instruments. In the combined use of 

 ammeter, wattmeter and voltmeter, the best method to use depends 

 somewhat upon the conditions of the test. The arrangement of Fig. I 

 is, for most purposes, as good as any; no corrections are made. 

 In the short-circuit test of a transformer, the reading of the current 

 is most important; hence, Fig. 6, Exp. 5-B, the ammeter for this test 

 can best be connected on the load side of the other instruments. For 

 the open-circuit test, voltage is important and not current; the 

 ammeter is, therefore, in the supply line and the instruments arranged 

 as in Fig. i, Exp. 5-B (requiring a correction) or as Fig. I of this 

 experiment (requiring no correction and hence simpler to use). See 

 3a, Exp. 5-B. 



45. Multipliers. To extend the range of a voltmeter, either a 

 series resistance (called a multiplier) or a potential transformer can 

 be used. The potential range of a wattmeter is extended in the same 

 way. 



To extend the range of an ammeter, a current transformer is used ; 

 the primary of the transformer is connected in series with the line, 

 the secondary being short-circuited through the ammeter. The cur- 

 rent range of a wattmeter is extended in the same way. 



The ratio of transformation of any potential or current trans- 

 former must be accurately known, and, for a current transformer, 

 this ratio must be known in connection with the particular instru- 

 ment and secondary leads with which it is to be used. Any small 

 phase shifting, due to the fact that primary and secondary quantities 

 are not exactly in phase opposition, introduces no error in the use 

 of instrument transformers with ammeters or voltmeters, but with 

 wattmeters such phase shifting may introduce considerable error and 

 needs to be taken into consideration for accurate work. For a com- 

 plete discussion, see Electric Measurements on Circuits Requiring 

 Current and Potential Transformers, a paper by L. T. Robinson, read 

 at the June, 1909, meeting of the A. I. E. E. 



