# When was it first noticed, or demonstrated, that radioactive material became warm?

Perusing the Radioactive Thermoelectric Generator or rtg tag in Space SE will show how important these items are for Space exploration both as a source of heat to keep spacecraft instrumentation warm, and to power the thermoelectric generators for electricity. The Curiosity rover on Mars uses it's RTG for both and includes a fluid circulation system to heat (or cool) various sections.

RTG's most often use an alpha decay source like Plutonium-238 or perhaps in the future Americium-241) rather than a fission source, to minimize longer range radiation that can damage the spacecraft.

Question: I'm not sure if the self-heating of a radioactive sample was first noticed then explained, or predicted and then demonstrated. But in either case, when was a measured or even noticed temperature rise first documented or reported?

• THere's a fine line between "gives off energy in the form of photons and particles" and "gets warm" , as both of those indicate energy release. – Carl Witthoft Jun 25 '18 at 12:03
• @CarlWitthoft Luckily language provides us with good clear words to make various distinctions clear. I think "warm" and "temperature" will do the trick in this case. – uhoh Jun 25 '18 at 14:34