478 ELECTRIC MEASUREMENTS [Cn. XIII 



take the example of an arc lamp which uses 20 amperes direct 

 current from a no volt line. The line then supplies 20 x no = 

 2,200 watts or 2.2 kilowatts. If this arc were used for only a few 

 minutes, the energy supplied would be comparatively small, but if 

 the arc were used all day, the energy supplied and hence the coal 

 or other fuel consumed in generating this power would be compara- 

 tively large. In order to measure this energy, the power measured 

 in kilowatts is multiplied by the time the power is used. In the 

 above example, if the arc were run for eight hours the electrical 

 energy used would be 2. 2x8 = 17.6 kilowatt-hours. 



ELECTRIC MEASUREMENTS: VOLTMETERS, AMMETERS, WATT- 

 METERS FOR DIRECT CURRENT 



662. Voltmeter for direct current. This is an instrument for 

 measuring in volts the difference of potential between two points 

 of an electric circuit. 



The voltmeter must be adapted to the kind of current direct 

 or alternating and for the pressure, low voltage or high voltage. 

 It consists of a delicate galvanometer of exactly the same type as 

 that for an ammeter, but it has a high resistance in series with it. 

 This high resistance allows but a small current to flow through the 

 galvanometer; and this small current is proportional to the differ- 

 ence of pressure or voltage between the binding posts of the volt- 

 meter, and causes the needle of the voltmeter to be deflected. 

 Numbers on the dial indicate the voltage for different amounts of 

 the deflection. 



663. Connection of the voltmeter with the circuit to be 

 measured. One pole of the voltmeter is positive and one negative. 

 To connect the instrument with the circuit for determining the 

 voltage between two points, the positive binding post of the volt- 

 meter is connected by a wire to the positive point in the circuit, and 

 the negative binding post with the negative point in the circuit 

 (fig. 272). This gives the full electric pressure between the two 

 points connected with the voltmeter, although only a very small 

 current flows through it on account of its high resistance. The 



