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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 comparatively 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, WATTMETERS 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 difference of pressure or voltage between the binding posts of the voltmeter, 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 voltmeter 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