Copyright Goodheart-Willcox Co., Inc. 376 Motors and Electric Control Systems There are only two types of voltage to measure: direct current (dc) voltage and alternating current (ac) voltage. A technician must know the type of current in a circuit in order to set the meter’s function switch to the correct type of voltage. In ac circuits, a voltmeter reads the volt- age without regard to polarity because the current is con- stantly changing directions. In dc circuits, the polarity of the measured voltage is affected by the side of the load on which each voltmeter lead is placed. To measure voltage polarity correctly, place the black lead on the side of the load closest to the negative terminal of the circuit’s power supply and the red lead on the side of the load closest to the positive terminal of the circuit’s power supply. By being familiar with a circuit’s wiring diagram, a technician can determine on which side of the load to connect each lead. Meter Checks Before taking any electrical measurements, there are several things to consider and check: • Measurement setting. • AC/DC (polarity). • Leads in the jacks. Setting the measurement variable is not difficult: A for amperage (current), Ω for ohms (resistance), and V for voltage. But for amps and volts, remember to choose the right type: ac or dc. Also, remember to check whether the leads are plugged into the proper jacks on the meter. Often, the red lead needs to be moved to another jack to measure amperage, Figure 17-2. Pro Tip Voltmeters are designed to measure different voltage ranges, from low voltage measured in microvolts to high voltage measured in megavolts. The following are com- mon units of voltage: 1 microvolt (μV) = 0.000001 V (1/1,000,000 of a volt) 1 millivolt (mV) = 0.001 V (1/1000 of a volt) 1 kilovolt (kV) = 1,000 V 1 megavolt (MV) = 1,000,000 V Before taking a voltage reading, ensure that the volt- meter’s leads are in the correct meter jacks and that the voltmeter’s range switch is set to the correct voltage scale. Most residential air-conditioning and refrigeration appli- cations use 120 V or 240 V of alternating current. Low- voltage circuits usually operate at 24 V. Electronic circuits, Voltmeter Polarity Because current is flowing in only one direction in a dc circuit, there is an unchanging polarity. Connecting an analog voltmeter’s leads incorrectly will make the needle try to move backward, which may damage the meter in high-voltage dc applications. Caution V3 V2 V1 Power source Light Switch Heating element Voltmeter 100V 120V 20V Goodheart-Willcox Publisher Figure 17-1. Voltmeters connected in parallel are measuring the voltage of the input power (V1), the voltage drop across a light (V2), and the voltage drop across a resistive heating element (V3). Volts Ohms Amps Milwaukee Electric Tool Corp. Figure 17-2. Match the measurement setting and the jacks into which the leads are connected.