Fig. 629.--Voltmeter connection for measuring the pressure in an electric circuit. The voltmeter is connected in parallel in the circuit at the point where the voltage is to be measured.
Fig. 630.--Voltmeter connection for measuring the "drop" or fall in voltage in a certain length of wire, as for instance, the length between the points A and B. The voltmeter is shunted between the two points whose pressure difference is to be measured.
Ques. What is the difference between a voltmeter and an ammeter?
Ans. A voltmeter measures pressure, while an ammeter measures current. As actually constructed, most voltmeters are simply special forms of ammeter.
If a high resistance be connected in series with a sensitive ammeter that will measure very small currents, then the current passing through the circuit is directly proportional to the pressure or voltage at its terminals and the instrument may be calibrated to read volts.
Ques. Explain the term "calibrate."
Ans. To calibrate a measuring instrument is to determine the variations in its readings by making special measurements, or by comparison with a standard.
Fig. 631.--Weston ammeter; view showing shunt enclosed within the instrument. Weston instruments are direct reading and dead beat. Although the scales have practically uniform divisions, it is not assumed in the calibration that they are uniform, and the scales are not printed or engraved. The method of calibration consists in laying out each large division of the scale by comparing the instrument with a standard, and then inking in the division lines so found. The smaller divisions between the large ones are then equally spaced and marked by a mechanical method.