The zero ohms control is a potentiometer for adjusting the 0 reading on ohmmeter functions. The reset is a circuit breaker and is used to protect the meter movement. Not all multimeters have this protection, but most have some sort of protection, such as a fuse. When the multimeter is not in use, it should have the leads disconnected and be switched to the highest voltage scale and ac. These switch positions are most likely to prevent damage if the next person using the meter plugs in the meter leads and connects them to a circuit without checking the function switch and the dc/ ac selector.

SCALES.- The numbers on the uppermost scale in figure 3-24 are used for resistance measurement. If the multimeter was set to the R x 1 function, the meter reading would be approximately 12.7 ohms. The numbers below the uppermost scales are used for de voltage and current. These numbers are also used with the scale just below them for ac voltage and current. The fourth scale from the top and the numbers just below the scale are used for the 2.5-volt ac function only. The lowest scale (labeled dB) is not discussed. Figure 3-24 shows how the given needle position should be interpreted with various functions selected.

Multimeter Safety Precautions

As with other meters, the incorrect use of a multimeter could cause injury or damage.

The following safety precautions are the minimum for using a multimeter:

1. De-energize and discharge the circuit completely before connecting or discon-necting a multimeter.

2. Never apply power to the circuit while measuring resistance with a multimeter.

3. Connect the multimeter in series with the circuit for current measurements, and in parallel for voltage measurements.

4. Be certain the multimeter is switched to ac before attempting to measure ac circuits.

5. Observe proper dc polarity when measuring dc

6. When you are finished with a multimeter, switch it to the OFF position, if available. If there is no off position, switch the multimeter to the highest ac voltage position.

7. Always start with the highest voltage or current range. Figure 3-24.- A multimeter scale and readings.

8. Select a final range that allows a reading near the middle of the scale.

9. Adjust the 0 ohms reading after changing resistance ranges and before making a resistance measurement.

10. Be certain to read ac measurements on the ac scale of a multimeter.

11. Observe the general safety precautions for electrical and electronic devices.

All valves are not constructed in the same manner. Therefore, the electrical tests performed on a valve should be accomplished as directed by the appropriate MIM. The voltage and resistance tests described in the following paragraphs are examples and should not be used as References for the performance of tests on aircraft.