CHAPTER II. 

 ALTERNATING CURRENT MEASUREMENTS. 



11. The difference between direct-current and alternating-current 

 ammeters and voltmeters.* In direct-current ammeters and volt- 

 meters the force which acts upon the pointer and produces the 

 deflection may be any function whatever of the current or electro- 

 motive force, provided only that the force has a perfectly definite 

 value for a given current or electromotive force, thus ensuring 

 tha't the given current or electromotive force will produce * 

 definite deflection. 



In alternating-current ammeters and voltmeters, on the other 

 hand, the force tending at any instant to move the pointer is pro- 

 portional f to the square of the current or electromotive force at 

 that instant, so that the average force, upon which the steady de- 

 flection depends, is proportional to the average square of the cur- 

 rent or electromotive force. Alternating-current ammeters and 

 voltmeters really measure, therefore, the average value of the 

 square of an alternating current or electromotive force. 



The standard method of calibrating an alternating-current 

 ammeter or voltmeter is by using direct currents or direct 

 electromotive forces, and the most reliable alternating-current 

 ammeters and voltmeters are those which indicate effective 

 values of alternating current or electromotive force correctly 

 when so calibrated, irrespective of frequency and wave form. 



Consider, for example, an ammeter in which the deflecting 



* All voltmeters, with the exception of the electrostatic voltmeter, are essentially 

 ammeters. That is, the electromotive force to be measured produces a current, which 

 actuates the pointer ; and the scale, instead of being arranged to indicate the value of 

 the current, is arranged to indicate the value of the electromotive force between the 

 terminals of the instrument. 



f This is intended only as a general statement and the qualification necessary to 

 make it exact would tend to make it unintelligible. See Art. 15. 



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