A simple probe made from two kinds of wire (i.e. platinum, rhodium) welded together. This probe is wired to a sensitive electronic meter that displays a reading of the voltage it generates when heated.
However the world of high temperature measurement and thermocouples is a complex one. There are many kinds of thermocouples. Some generate a nice smooth voltage increase that bears a direct relationship to temperature increase, others require complex software to make the translation. There are also different manufacturing processes, calibration techniques, response to different atmospheres, abilities to measure different temperature ranges, different types decay in their accuracy in different ways, variations in frequency of need for recalibration, etc.
Maintaining accurate pyrometers can be expensive and typical inexpensive type K devices used in potters kilns are not accurate at higher temperatures (most potters won't pay for the platinum/ 10% platinum-rhodium (type S) thermocouples and control systems that really should be used, and the more expensive plated switches and contacts). However the type K are more resistant to oxidation than types E, J, and T at temperatures over 500C.