An isotopic thermoelectric generator was developed in the US by Numec Corporation under a contract from the US Atomic Energy Commission and sold for $3,200 (back in 1974). The thermopile consisted of doped bismuth telluride pairs that were placed in a parallel/series arrangement to generate some 300 μW of power to run this Arco Medical model NU-5F pacemaker.
The nuclear battery in the ARCO Medical device used a tiny 8 Ci slug of metallic Plutonium 238 (Pu-238). The radiation produced by the Pu-238 bombarded the walls of its container, producing heat that a thermopile then converted to an electrical current. A thermopile is a stack of thermocouples, which are devices that convert thermal energy directly into electrical energy using Seebeck effect. The thermocouple was made of two kinds of metal (or semiconductors) connected to each other in a closed loop. If the two junctions are at different temperatures, an electric current will flow in the loop.
NOTE: The photograph of the NU-5F pacemaker was part of the media kit distributed by Arco Medical in 1974. I do not personally possess any live atomic pacemakers or RTGs.
Click here for my paper on nuclear-powered pacemakers.
Click here for the authoritative paper on nuclear batteries for pacemakers: Fred N. Huffman, et al., RADIOISOTOPE POWERED CARDIAC PACEMAKERS, Cardiovasc Dis., 1(1): 52–60, 1974. (Copyright notice: This article is free from www.pubmedcentral.nih.gov).
Click here to find out what to do if you find a nuclear pacemaker.