Copyright Goodheart-Willcox Co., Inc. 378 Motors and Electric Control Systems in either wire represents the current draw of the electrical load. If a technician places the clamp-on ammeter around both the hot and neutral wires, the currents will cancel each other out, and the reading will be zero. In-Line Ammeters An in-line ammeter is an ammeter that is connected into a circuit with leads to measure the circuit’s current. An in-line ammeter’s inner circuitry has low-resistance shunt resistors placed across its terminals. These shunt resistors protect the ammeter’s sensing coil and allow it to measure high levels of current. Unlike voltmeters and ohmmeters, in-line ammeters are connected in series (in-line) with the circuit or load being tested, Figure 17-6. In a dc circuit, verify that the ammeter’s leads are con- nected correctly to match the polarity of the power source before energizing the circuit. The black lead is placed on the side of the load closest to the negative terminal of the Obtaining Low-Amperage Readings There may be circuits in which the current is too low for a clamp-on ammeter to accurately measure. If the wire is long enough for free movement, wrap the wire multiple times around the jaws to get a higher reading. Divide the reading by the number of times the wire is wrapped. For example, if you wrap the wire around the jaws ten times, divide the reading by ten to convert to the actual current level. Most wires are not long enough to wrap ten times, so some technicians carry a ten-loop jumper wire to help measure low current. Wire wrapping is not recommended for wire that is 10 AWG or larger. Pro Tip Milwaukee Electric Tool Corp. Figure 17-5. A clamp-on ammeter is clamped around only one conductor of a particular circuit to determine the circuit’s current draw. Movable jaws Digital display Jaw lever Amprobe Figure 17-4. A digital clamp-on ammeter measures a conductor’s current draw when the movable jaws are closed around the conductor being tested. Switch 2A 2A 2A In-line ammeter Heating element Light Power source A3 A1 A2 Goodheart-Willcox Publisher Figure 17-6. An in-line ammeter is connected in series with the circuit or load being measured. Because this is a series circuit, there is only one path for current to travel, so each ammeter will have the same reading.