POWER AND POWER-FACTOR. 83 



In the case of both 3-voltmeter and 3-ammeter 

 measurements, the power is usually calculated directly 

 by the use of the formulae given, and not by construction 

 of a triangle, or by the calculation of cos <f>. 



It will be seen that in order to measure the power in 

 an inductive circuit by the 3-voltmeter method it is 

 necessary to insert a non-inductive resistance in the 

 circuit, and thus to absorb a considerable proportion of 

 the power during the measurement. 



In making a measurement with three .ammeters it is 

 necessary to have a branch circuit which takes a large 

 part of the total power. 



For these reasons it is not usual to employ these 

 methods except in such cases as the measurement of the 

 load of a machine under test where the manner in which 

 the power of the circuit is absorbed is not of importance. 

 For continuous reading of the power in a circuit employed 

 in doing useful work the method is not admissible. 



In carrying out the experiments just given, if fairly 

 small currents are used, the losses in the voltmeters, 

 ammeters, and wattmeter shunt coils may be found to 

 seriously affect the accuracy of the measurement. A 

 knowledge of the resistance of each instrument is 

 necessary to determine the best arrangement for mini- 

 mising these errors, and the experiments, if carefully 

 carried out, form a valuable exercise in judgment as to 

 their use. 



