To check electronic parts, use a multimeter. Multimeters are instruments that allow the user to test or measure electronic components. Advanced ones check properties such as capacitance and transistor gain, but even the most basic ones are able to measure resistance, voltage and current. Resistance measurements must be made with the current off. Voltage and current measurements are made with the current on, but care must be taken to use DC or AC settings. Additionally, voltage measurements must be made with the multimeter added parallel to the circuit, while current measurements must be made with the multimeter added in series with the circuit.
Measure the resistance of the resistor. Do this by turning the multimeter on, and placing the knob on the resistance setting, which is normally represented by the Greek letter omega. Place the red probe of the multimeter on one end of the resistor, and place the black probe on the other. Record the value of the resistance, which will be approximately 1k. A representative value is 982 ohms.
Create an LED circuit by placing one in series with the resistor and the two batteries. Do this by connecting the red lead of the battery holder to the circuit board. Insert the resistor, and wire one of its ends to the battery holder's red lead. Connect the other end of the resistor to the positive terminal of the LED. Attach the negative end of the LED to the black lead of the battery holder.
Insert the batteries into the holder. The LED will light.
Use the multimeter to measure the voltage across the resistor. First, make sure that the switch is on DC voltage, and not AC. Turn the knob on its body to the voltage setting, which is indicated by the letter "V." It must be at least 4 volts. Place the multimeter in parallel to the resistor by placing the red probe on one of its terminals, and the black probe on the other. Record the voltage. If the reading is negative, reverse the probes. For a 9-volt battery, a representative reading is 6.42 volts.
Measure the voltage across the LED by placing the red probe on the positive side and read the volts. For a 9-volt battery, a representative reading is 2.88 volts.
Turn the multimeter off, and place the knob on the multimeter on the current setting, which is usually indicated by the letter "A." Remove the red probe from the voltmeter opening on the casing, and insert it into the one for current measurements, which is also labeled with an "A."
Find the current through the LED by placing the multimeter in series with the multimeter. Detach the wire that connects the negative terminal of the LED to ground. Add a wire to the negative side of the LED. Use an alligator clip and attach the wire to the red probe of the multimeter. Use another alligator clip to connect the black probe of the multimeter to the black lead of the battery holder. Turn the multimeter on, and record the current. For a 9-volt battery, a representative reading is 6.45 mA or milliamps.
When taking current measurements, never connect the multimeter directly to the battery. This will form a short circuit, and the multimeter will blow a fuse.