Where should a voltmeter be connected when testing a device?

When testing a device with a voltmeter, it is important to connect it in parallel with the device being tested. This method allows the voltmeter to measure the voltage across the device without altering the circuit's operation. By connecting in parallel, the voltmeter taps into the same potential difference that the device experiences, providing an accurate reading of the voltage.

Connecting a voltmeter in series, as would be suggested in one of the other options, would cause it to be part of the circuit's current flow, which could lead to incorrect readings and potentially damage the voltmeter. This is because a voltmeter has a very high internal resistance designed to minimize its impact on the circuit, but if placed in series, it will disrupt the normal operation of the device being tested.

Testing at the main panel could also provide information about the incoming voltage supply to the system but does not specifically tell us about the voltage across the individual device. Therefore, connecting the voltmeter in parallel is crucial for accurate voltage readings associated with the device itself.