What might a defective voltmeter show when measuring voltage?

A defective voltmeter can show fluctuating values when measuring voltage primarily due to issues such as poor connections, internal faults, or damaged components. These irregularities can cause the voltmeter's readings to vary continuously, reflecting an unstable measurement environment.

Fluctuating values might be the result of electrical interference, weak batteries, or a malfunction in the device itself, which makes it unable to provide a stable output. This behavior is particularly problematic when precise voltage readings are needed, as it obscurely represents the actual voltage present in the circuit being measured.

Other options, such as consistently zero readings or only positive values, imply specific faults that could occur but are less commonly associated with general defects in a voltmeter. The ability to fluctuate between values captures a broader range of potential issues that could arise from a malfunctioning instrument, highlighting the nature of operational failure in measuring devices.