42 ELEMENTS OF ELECTRICAL ENGINEERING. 



average force action, k x average e 2 , due to the alternating elec- 

 tromotive force e, which gives the same deflection as S, that is, 

 ks 2 = k x average e 2 , or & = i/ average e 2 . 



The great difficulty with the electrostatic voltmeter arises from 

 the excessively weak forces of attraction between two metal plates 

 with electromotive forces less than several hundred volts, so that 

 a low-reading electrostatic voltmeter must be very delicate, and 

 therefore unsuited to the conditions of practical use. For high 

 electromotive forces, however, the electrostatic voltmeter is very 

 satisfactory. 



One form of electrostatic voltmeter, the so-called absolute elec- 

 trometer,* does not need to be calibrated, inasmuch as it gives 

 the value of the electromotive force directly in terms of purely 

 mechanical measurements. 



15. Plunger type ammeters and voltmeters. In instruments 

 of this type the current flows through a coil of wire which mag- 

 netizes and moves a pivoted or suspended piece of soft iron to 

 which the pointer is attached. 



Plunger ammeters. In this type of instrument the force tend- 

 ing to move the pivoted piece of iron is not proportional to the 

 square of the current, and therefore such an instrument does not 

 indicate effective values of alternating current when it has been 

 calibrated with direct current. It is necessary therefore to cali- 

 brate a plunger type ammeter with alternating current which, for 

 the purpose of the calibration, is measured by an electro-dyna- 

 mometer ammeter. 



Plunger type voltmeters. An instrument which will indicate 

 effective alternating current correctly when calibrated as an am- 

 meter with direct current, will also indicate effective alternating 

 electromotive force correctly when calibrated as a voltmeter with 

 direct electromotive force, provided the inductance of the instru- 

 ment is negligible. This is exemplified by the use of the electro- 

 dynamometer as a voltmeter as explained in Art. 1 3. The plunger 

 type instrument, however, does not indicate effective alternating 



*See Elements of Physics, Nichols and Franklin, Vol. II, pages 183-184. 



