Copyright Goodheart-Willcox Co., Inc. Chapter 10 How Electric Meters Work 181 10.2.4 Resistance Measurement The meter is configured to supply power to the component under measurement, Figure 10-16. Current flows from the neg- ative battery terminal through the component under test and back to the meter’s A/D converter and shunts. The analog bat- tery voltage is also digitized. This information is processed and sent to the display unit. Fundamentally, the microprocessor is calculating Ohm’s law (R = E/I). In the figure illustration, the meter battery volt- age is 3 V. Since 1 A flows through the resistor, the resistor value is 3 Ω. As with current measurement, shunts are with resistance measurement to reduce current flow through the A/D. Connecting the probes together should yield 0 Ω. Several factors can generate small resistances. It is common to see a reading of 0.1 Ω to 0.2 Ω. These factors are test lead wire resis- tance and contact resistance of the test leads to the meter and internal meter calibration. Mode Switches Ensure that mode switch is set to voltage. Checking voltage in other functions can damage the meter, which, in turn, may injure the user. Safety Note Phantom Voltage When measuring voltage on an unpowered circuit, the meter may display a voltage. The value can be high enough to cause confusion of its validity. This is called phantom voltage. It is caused by capacitance developed from nearby live wires. The unpowered wires act like capacitor plates. Some meters offer an optional feature to filter out phantom voltage. Pro Tip Load 10 MΩ Meter 120.0 V 119.99 V 0.01 V 1 KΩ Load N L A Load 120.00 V 1.2 V 118.8 V 10 MΩ Contacts 1 KΩ Load N L B Meter 10 MΩ Goodheart-Willcox Publisher Figure 10-15. A—Voltage is measured across an open switch. All the current flows through the meter, and it measures close to line voltage. B—Voltage is measured across a closed switch. Most of the current flows through the closed switch. A very small current flows through the meter resulting in a close to zero volts displayed. Shunts Microprocessor 3.00 Ω A/D Fuse Resistor 1 A 3 V Resistance R = E / I Goodheart-Willcox Publisher Figure 10-16. The digital meter is configured to measure resistance.