# Module 3 - Introduction to Circuit Protection, Control, and MeasurementNavy Electricity and Electronics Training Series (NEETS) Chapter 1:  Pages 1-21 through 1-30

Notice that the meter movements shown in table 1-1 will indicate .029 volts to .I volt at full scale, and the sensitivity ranges from 1000 ohms per volt to 200,000 ohms per volt. The higher sensitivity

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meters indicate smaller amounts of voltage. Since most voltage measurements involve voltage larger than .1 volt, a method must be used to extend the voltage reading.

Figure 1-28 illustrates the method of increasing the voltage range of a voltmeter.

Figure 1-28. - A voltmeter and a range resistor.

In figure 1-28(A), a voltmeter with a range of 10 volts and a resistance of 1 kilohm (R2) is connected in parallel to resistor R1. The meter has .01 ampere of current (full-scale deflection) and indicates 10
volts. In figure 1-28(B), the voltage has been increased to 100 volts. This is more than the meter can
measure. A 9 kilohm resistor (R3) is connected in series with the meter (R2). The meter (R 2) now has .01 ampere of current (full-scale deflection). But since R3 has increased the voltage capability of the meter, the meter indicates 100 volts. R3 has changed the range of the meter.

Voltmeters can be constructed with several ranges by the use of a switch and internal resistors. Figure 1-29 shows a voltmeter with a meter movement of 100 ohms and 1 milliampere full-scale deflection with 5 ranges of voltage through the use of a switch. In this way a voltmeter can be used to measure several different ranges of voltage.

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Figure 1-29. - A voltmeter with internal range resistors.
The current through the meter movement is determined by the voltage being measured. If the voltage measured is higher than the range of the voltmeter, excess current will flow through the meter movement and the meter will be damaged. Therefore, you should always start with the highest range of a voltmeter and switch the ranges until a reading is obtained near the center of the scale. Figure 1-30 illustrates these points.

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