Which instrument is best to measure a small gap between relay contacts?

Using a feeler gage is the most appropriate choice for measuring a small gap between relay contacts due to its design and purpose. A feeler gage consists of a collection of thin metal strips of varying thickness, allowing for precise measurements in small increments. When inserted between the contacts, it can help determine the exact width of the gap, ensuring that the relay operates within the desired specifications.

The other instruments listed do not serve this specific purpose. An infrared lamp is primarily used for heating and does not measure gaps. A watt meter measures electrical power and would not provide any information about the physical distance between contacts. A hacksaw, used for cutting materials, is not intended for measurement and would be inappropriate for this application. Hence, the feeler gage stands out as the suitable instrument for accurately assessing small spaces such as the gap between relay contacts.