MULTIMETERS Up to this point, each of the instruments we have discussed, for the most part, performs only one function. The exception was the clamp-on ammeter/ ohmmeter. In a similar way the analog meters and digital meters perform several (or multiple) functions and are therefore referred to as multimeters.

An analog instrument usually makes use of a needle to indicate a measured quantity on a scale. Digital meters indicate the quantity directly in figures. We will discuss both types here because you will use both types.

Notice that each multimeter in figure 7-24 (A, B, C, and D) consists of a case to enclose the indicating device, one or more functions and/ or range switches, and internal circuitry and jacks for external connections.

Voltage Measurements Before plugging the test leads into the jacks, set the switches for the measurement. Let's look at anexample. You are about to measure the voltage at a standard wall outlet in an office. You already know from experience that the voltage should be in the area of 115 to 125 volts ac. You have one of two types of multimeters-an analog meter or a digital meter. Because you do know the voltage to be tested, you would set the function switch to AC and the voltage to 250V. For the operation of the range and function switches on the particular meter, you should check the manufacture's literature.

What should you do if you have no idea what the voltage is? There are times when you should not get

Figure 7-24.- Typical multimeters (analog types A and B and digital types C and D).

near the equipment; in this case, you should check with someone who knows (for example, a public works engineer or line crew supervisor). Check the highest range on your instrument. If you have a meter and you know the voltage value should not exceed 1,000 volts ac, then set the range/ function switch to 1,000 ACV.

WARNING The following sequence of steps is important for your safety. Stay alert and follow them carefully.

Plug the test leads into the appropriate jacks for the test you are about to perform. When you have red and black test leads, get into the habit of using the black lead with the common or -(negative) jack, even when measuring ac volts. For either analog meter, plug the red lead into the + (positive) jack. With either of the digital meters, use the jack marked "V-O" (volts-ohms).

Connect the two test leads to the two conductors/ terminals of the wall outlet while holding the insulated protectors on the test leads. Do not touch the probes or clips of the test leads. Take the reading. If you have the meter range switch at the highest setting and see that the voltage value is within a lower voltage range, set the range switch to the lower range that is still higher than the voltage reading you remember. When you take a reading at a higher range and switch to a lower range, the reading at the lower range will be more accurate. Be certain to read from the scale that matches the range setting of the switch; for example, when using the multimeter with the switch set to 300 AC VOLTS, read from the scale that has a maximum reading of 300 ac. Simply take the reading directly from either of the digital multimeters.

WARNING Always be alert when taking voltage or amperage measurements if it is necessary to move the meter. If the instrument is moved in a way that causes tension on the test leads, one or both leads may be pulled Tom the jack( s). The leads will be energized just as the circuit to which they are connected, and they can be dangerous.

The positions of the jacks may differ for a particular measurement, from one meter to another. Notice how the jacks are labeled on the instrument you use, and follow the instructions from the manufacturer of the instrument.