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Amplified Resistance Measurements

The electronic meter is used for resistance measurements in a manner similar to the way it is used for current measurements. Using the same types of amplifier stages as was used for the voltmeters and ammeters, where a 0.5-volt-signal produced a full-scale deflection, resistance circuits can be set up using standard battery source voltages supplying current to standard high­ precision resistance circuits, whose values are known. When the resistor under test is connected into the circuit, it will change the total resistance, resulting in a change in current flow, which produces a signal voltage that is directly related to the resistance under test.

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Amplified Current Measurements

To use the meter movement to make voltage and resistance measurements, the use of Ohm’s law is required so the current flow reading can be interpreted in terms of voltage or resistance. The amplified analog meter differs because the amplifier is voltage sensitive. As explained for amplified voltage measurements, a high input resistance is used with a range switch to tap down the voltage applied to the input of the amplifier stage. Since the amplified meter is a voltage sensitive device, the input circuit used for current measurements must convert the current to corresponding voltage levels and use Ohm’s law to interpret the related current flowing in the circuit under test.

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Amplified Input Voltmeter

The higher the ohms/volt rating of a voltmeter, the less the voltmeter will upset circuit conditions. And the less circuit conditions are upset, the more accurate the reading will be. Most of the higher-end voltmeters and multimeters available now are rated at about 20,000 ohms/volt; more accurate voltmeters are rated at 100,000 ohms/volt. In some of the high-resistance circuits found in some present-day equipment, however, even a meter rated at 20,000 ohms /volt will greatly upset circuit conditions, and result in an incorrect reading. While a 100,000 ohms/voltmeter will give more accurate readings, even more accuracy is needed with some circuits. To overcome this problem, a device with a high ohms/volt rating called an electronic voltmeter was developed.

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