Measuring Resistance

## Measuring resistance

To measure resistance with an analog ohm meter[1], first touch the probes of the meter together and adjust the zeroing knob until the meter reads zero ohms. Once the meter is zeroed, place the probes across the resistor to be measured. If the reading is too high or too low to be meaningful, adjust the range of the meter. A digital ohmmeter doesn’t require zeroing as an analog meter does.

The resistance function of a multimeter is also used to measure continuity. This is useful for checking for breaks in wires or blown fuses. A good wire or fuse should read zero ohms where a bad wire or fuse will read infinite ohms. Many meters have a continuity function. This sounds a buzzer when there is continuity.

Caveats:

An ohmmeter should never be connected to a circuit that has power applied.

When measuring very high resistances be sure you are not touching the probes. Your body has a resistance of several hundred thousand ohms. When measuring a low value of resistance, this doesn't make much difference. However, when measuring something like 1,000,000 ohms (1 Megaohm), the reading will be affected by your own resistance.

Analog meters usually have an “off” position. This shorts the inputs to the movable coil together. As explained in DC motors below, this dampens the movement of the needle when the meter is shaken, protecting it from damage when the meter is bumped around when not in use. If you have an analog meter, you can demonstrate this. Shake the meter lightly and observer the needle movement. Now put the switch into the “off” position and shake it again. The needle will move much less.

Measuring Resistance

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