46 ELEMENTS OF ELECTRICAL ENGINEERING. 



A given direct-current ammeter may be adapted to read the 

 value of a large current by connecting a low resistance shunt 

 across the terminals of the instrument. The use of such a shunt 

 with an alternating -current ammeter, however, is open to the 

 objection that the multiplying value of the shunt varies with the 

 frequency, so that a shunted alternating-current ammeter should 

 be calibrated by alternating current of the same frequency as that 

 for which the instrument is to be used. 



The so-called "potential transformer" is a small transformer for 

 stepping down a high voltage in a known ratio so that the high 

 voltage may be measured by a low reading voltmeter. Most 

 switch-board voltmeters in alternating-current generating stations 

 are connected to the mains through step-down " potential " trans- 

 formers. 



The so-called "current transformer" is a small transformer 

 through whose primary coil the current to be measured flows, 

 and whose secondary coil is short-circuited through an ammeter. 

 The ratio of primary to secondary current is known, so that the 

 value of the primary current may be inferred from the ammeter 

 reading. Most switch-board ammeters in alternating-current 

 generating stations are connected through " current " transformers 

 in this way. 



The use of the potential transformer and of the current trans- 

 former makes it possible to completely insulate the switch-board 

 instruments from high-voltage mains. 



18. Measurement of power in alternating current circuits. Defi- 

 nition of power factor. The power delivered to a circuit by a 

 direct-current generator is equal to the product of the voltage 

 and the current, so that the power delivered by a direct-current 

 generator can be determined by a voltmeter and an ammeter. 

 The power delivered by an alternating-current generator, on the 

 other hand, is generally less, and often very much less, than the 

 product of the effective electromotive force of the generator and 

 the effective current, so that the power delivered by an alternating- 



