Which of the following can be used to display signal strength on a numeric scale?

A meter is the appropriate choice for displaying signal strength on a numeric scale because it is specifically designed to provide an analog or digital readout of electrical quantities, including voltage, current, and signal strength. Meters can reflect varying levels of signal intensity, allowing operators to assess the signal quality and performance in real time.

The other options serve different functions that do not directly correlate with the task of displaying signal strength. A potentiometer is primarily used to adjust levels of resistance in a circuit but does not provide a numeric readout of signal strength. A transistor is a semiconductor device used for amplification and switching, not for measurement. A relay, on the other hand, is an electromechanical switch that opens and closes circuits but does not measure or display signal characteristics.

Thus, a meter is essential for monitoring signal strength effectively in ham radio operations.