What device is used to measure the resistance or breakdown strength of conductor insulation?

The correct answer is a megohmmeter, which is specifically designed to measure high resistance values, particularly the insulation resistance of conductors. This tool applies a high voltage to the insulation and measures the resultant current flow, allowing technicians to determine how effective the insulation is at preventing current leakage. In contexts involving electrical safety, it helps assess whether the insulation is intact or if it has degraded to a point where it could lead to short circuits or electrical shocks.

Other measuring devices do not serve the same purpose. For instance, a voltmeter measures voltage, and while it can be part of troubleshooting, it is not suitable for assessing insulation integrity. An ammeter measures current flow in a circuit, which is not relevant for insulation testing. A circuit tester, typically used to determine the presence of voltage in a circuit, lacks the necessary capability to assess the resistance or breakdown strength of insulation, making it ineffective for this specific measurement.