This is plutonium. Pu-238 to be exact. It is radioactive, that means it releases heat just sitting there. This chunk releases about 62 watts. This property is useful if you need electricity in locations which are very inaccessible. Like remote arctic coasts, or space. You see, you attach the hot end of a thermocouple to this chunk of plutonium and the cold end to a radiator. The temperature gradient causes electrons to flow, making electricity. These devices are called Radioactive Isotope Thermoelectric Generators, or RTGs for short.