Final answer:
The red probe of a voltmeter should be placed at the positive terminal of the voltage source first when troubleshooting a circuit. Other points are then tested to locate a voltage drop that indicates a broken wire. The voltmeter is used in parallel to avoid affecting the circuit.
Step-by-step explanation:
When using a digital voltmeter to troubleshoot a suspected broken wire in a circuit, the correct procedure is to first place the red probe (the positive probe) at the point where you expect voltage to be present, and then proceed to test other points to find where the voltage drops off or changes. This initial point is often the positive terminal of the voltage source, such as the battery. By placing the voltmeter in parallel with the circuit components you suspect might be broken, you can measure the potential difference or voltage across them. If the voltmeter reads zero or a significantly lower voltage than expected, it could indicate a break in the wire at some point before the probe. However, one should always be careful to ensure the voltmeter itself does not affect the circuit, which is why voltmeters are designed with high resistance to minimize current flow through them.