The arc-shaped marks on the meter dial face may be different colors that indicate each scale, so they will have different values. These determine the ranges of magnitude. A wider mirror-like surface shaped like the scales might also be present. The mirror is used to help reduce what’s called “parallax viewing error,” by lining up the pointer with its reflection before reading the value the pointer is indicating. In the image, it appears as a wide gray strip between the red and black scales. Many newer multimeters have digital readouts, rather than the analog scale. The function is basically the same, you’ll just get a numerical readout.
Some meters have an “Off” position on this selector switch while others have a separate switch to turn the meter off. The meter should be set to “Off” when stored and not in use.
One is usually labeled “COM” or (-), which stands for for common. This is where the black test lead will be connected. It will be used for nearly every measurement taken. [3] X Research source The other jack or jacks should be labeled “V” (+) and the Omega symbol (an upside down horseshoe) for Volts and Ohms, respectively. The + and symbols represent the polarity of probes when set for and testing DC volts. If the test leads were installed as suggested, the red lead would be positive as compared to the black test lead. This is nice to know when the circuit under test isn’t labeled + or, as is usually the case. Many meters have additional jacks that are required for current or high-voltage tests. It is equally important to have the test leads connected to the proper jacks as it is to have the selector switch range and test type (volts, amps, ohms) set. All must be correct. Consult the meter manual if you’re unsure which jacks should be used.
The meter may have more than one battery and they may be of different sizes. A fuse is provided to help protect the meter movement. Likewise, there is often more than one fuse. A good fuse is required for the meter to function, and fully charged batteries will be required for resistance/continuity tests.
Rotate the knob slowly to move the needle as close to the 0 position on the Ohms scale as possible. If new batteries are installed, this should be easy to do a needle that will not go to zero indicates weak batteries that should be replaced. [4] X Research source
Find the Ohm scale on the dial. It is usually the top-most scale and has values that are highest on the left of the dial (“∞” or a sideways “8” for infinity), gradually reducing to 0 on the right. This is opposite of the other scales, which have the lowest values on the left and increase going right. [5] X Trustworthy Source Science Buddies Expert-sourced database of science projects, explanations, and educational material Go to source
Set the range (if provided) to R x 100.
Note that this position is the “short circuit” or “zero ohms” indication for this R x 1 range of this meter. Always remember to “zero” the meter immediately after changing resistance ranges or you’ll get a faulty reading. If you’re unable to obtain a zero ohm indication, this may mean the batteries are weak and should be replaced. Retry the zeroing step above again with fresh batteries.
Have a helper hold the bulb by the glass only. Press the black probe against the threaded base and the red probe against the center tab on the bottom of the base. Watch the needle move from resting at the left and move quickly to 0 on the right.
In the previous step, each number represented a value that was 100 times greater. Thus, 150 really was 15,000 before. Now, 150 is just 150. Had the R x 10 scale been selected, 150 would have been 1,500. The scale selected is very important for accurate measurements. With this understanding, study the R scale. It is not linear like the other scales. Values at the left side are harder to accurately read than those on the right. Trying to read 5 ohms on the meter while in the R x 100 range would look like 0. It would be much easier at the R x 1 scale instead. This is why when testing resistance, adjust the range so that the readings may be taken from the middle rather than the extreme left or right sides.
Loosely hold a probe in each hand and read the meter. Squeeze both probes tightly. Notice the resistance is reduced. Let go of the probes and wet your hands. Hold the probes again. Notice that the resistance is lower still.
Testing round cartridge type and older style glass automotive fuses will indicate low values of resistance if the fuse is lying on a metal surface when under test. The meter indicates the resistance of the metal surface that the fuse is resting upon (providing an alternate path between the red and black probe around the fuse) instead of trying to determine resistance through the fuse. Every fuse in this case, good or bad, will indicate “good,” giving you a faulty analysis.
If the meter were set to the 50 volt range and a common U. S. electrical outlet were to be tested, the 120 volts present could irreparably damage the meter. Start high and work downward to the lowest range that can be safely displayed.
The maximum value scale should coincide with selector knob ranges. The voltage scales, unlike the Ohm scales, are linear. The scale is accurate anywhere along its length. It will of course be much easier accurately reading 24 volts on a 50 volt scale than on a 250 volt scale, where it might look like it is anywhere between 20 and 30 volts.
Press the black probe into one of the straight slots. It should be possible to let go of the black probe, as the contacts behind the face of the outlet should grip the probe, much like it does when a plug is inserted. Insert the red probe into the other straight slot. The meter should indicate a voltage very close to 120 or 240 volts (depending on type outlet tested).
If the pointer did not move, it is likely that DC was chosen instead of AC. The AC and DC modes are not compatible. The correct mode must be set. If not set correctly, the user would mistakenly believe there was no voltage present, which could be a dangerous mistake. Be sure to try both modes if the pointer does not move. Set meter to AC volts mode, and try again.
Be aware that most multimeters will only measure extremely small amounts of current, in the uA and mA ranges. 1 uA is . 000001 amp and 1 mA is . 001 amp. These are values of current that flow only in the most delicate electronic circuits, and are literally thousands (and even millions) of times smaller than values seen in the home and automotive circuits that most homeowners would be interested testing. Just for reference, a typical 100W/120V light bulb will draw . 833 Amps. This amount of current would likely damage the meter beyond repair.
To do this, insert the black probe into the “COM” or “-” jack and insert the red probe into the “A” jack. Shut off power to the circuit. Open the portion of the circuit that is to be tested (one lead or the other of the resistor). Insert the meter in series with the circuit such that it completes the circuit. An ammeter is placed in series with the circuit to measure current. It cannot be placed “across” the circuit the way a voltmeter is used (otherwise the meter will probably be damaged). Observe the polarity. Current flows from the positive side to the negative side. Set the range of current to the highest value. Apply power and adjust range of meter downward to allow accurate reading of pointer on the dial. Do not exceed the range of the meter, otherwise it may be damaged. reading of about 2 milliamps should be indicated since from Ohm’s law I = V/R = (9 volts)/(4700 Ω) = . 00191 amps = 1. 91 mA.
