Run the thermocouple wires to a PID controller with a relay or triac output that controls the gas solenoid valve. This results in tight control and high yield. Here, a deep annular groove is cut in the flat end of the drum. A stationary RTD probe is inserted parallel to the drum's axis so that it senses only the surrounding metal temperature. A temperature controller picks up the signal and drives a stationary induction heater inside the drum.
Radio transmission is another common option.
The circuit is powered by noncontact electromagnetic coupling. When you run out of the many ways of sensing the untouchable with a thermocouple or RTD, you can turn to sensing the infrared radiation from your process. Construction of an Infrared Thermometer Take a metal tube, put a lens in the end and a silicon radiation detector behind it. As the target gets hotter, the voltage increases. Not bad so far, but consider what the professionals do to make it useable.