202 ELEMENTS OF ELECTRICAL ENGINEERING. 



the resistance of the instrument. There is but one type of volt- 

 meter, namely; the electrostatic voltmeter, which is not essentially 

 an ammeter. In this instrument the electromotive force to be 

 indicated is connected to two insulated metal plates. The plates 

 then attract each other, and one plate which is delicately sus- 

 pended carries a pointer which plays over a divided scale. 



From the above statement it might seem that there is no differ- 

 ence between ammeters and voltmeters, whereas the failure on the 

 part of a novice to appreciate the very important difference leads 

 to many serious accidents in the laboratory. An ammeter, includ- 

 ing its shunt, always has a very low resistance, so that when it is 

 placed in a circuit the current may flow through the instrument 

 with the least possible opposition due to its resistance. A volt- 

 meter, on the other hand, is always designed to give its full deflec- 

 tion with a very small current, and its resistance is therefore 

 always high. When the instrument is connected to the terminals 

 of a dynamo or to supply mains the current which it takes is so 

 small that its effect on the system is negligibly small. 



The tangent-galvanometer type of voltmeter is seldom used. 

 The D'Arsonval galvanometer type is frequently used for direct 

 current circuits. The electrodynamometer type of voltmeter is 

 used for precision measurements of alternating voltages. The 

 plunger type is most frequently used for switch-board instru- 

 ments, especially for alternating currents. 



Voltmeter multiplying coils. If an auxiliary resistance be con- 

 nected in series with a voltmeter, the instrument still indicates the 

 electromotive force between its terminals, and the total electro- 

 motive force which is forcing current through the auxiliary resist- 

 ance and the instrument is greater than the electromotive force 

 indicated by the instrument in the ratio (* -f R}jr, where r is the 

 resistance of the instrument and R is the auxiliary resistance. 

 This auxiliary resistance is called a multiplying coil. Multiplying 

 coils are frequently used in this way to adapt a low reading volt- 

 meter to higher voltages. 



91. The wattmeter. Power delivered by direct-current mains 



