A good thermocouple should read approximately how many millivolts on a D.C. meter?

A good thermocouple typically produces a small voltage signal in the millivolt range, which is directly related to the temperature difference between its junctions. For many common types of thermocouples, the output voltage is generally around 40 microvolts per degree Celsius at standard reference conditions. When a thermocouple is functioning correctly at significant temperature differences, the output can indeed reach around 18 millivolts for a substantial temperature change.

This is consistent with the response of thermocouples, which, depending on their type, will have a specific output voltage for a given temperature change. Therefore, an output of approximately 18 millivolts reflects what one would expect from a properly functioning thermocouple under typical operational conditions, especially at higher temperatures.

In contrast, other options suggest outputs that are substantially higher than what is typical for thermocouples, reflecting either a misunderstanding of thermoelectric principles or misinterpretation of the voltage ranges used with more powerful electrical components rather than sensors like thermocouples.