The idea to incorporate radioactive material into spark plugs seems to belong to Alfred Hubbard who received a patent for this concept in 1929. The Firestone Tire and Rubber Company was, I believe, the only company to actually market the idea. Their first commercially available spark plugs became available in 1940.
Polonium-210 was incorporated into the electrodes that formed the spark-gap of the spark plug. More specifically, the polonium was added to the molten metal (a nickel alloy) from which the wires that were used to produce the electrodes were drawn. The alpha particles emitted by the decay of the polonium would ionize the gas within the spark gap and this would presumably result in a longer and/or “fatter” spark. The November 1941 issue of the Science Digest reported that tests had indicated that “30 percent fewer revolutions were required to start the motor as compared with other spark plugs.” According to the company’s advertising, the sparkplugs resulted in a “smoother motor performance . . . faster pick-up . . . quicker starting . . . save more gasoline.”
That there was any real benefit to using these spark plugs is somewhat questionable (other than the improved performance you get whenever you install new plugs). First of all, the half-life of the polonium-210, 138 days, meant that any effectiveness would be short-lived. Second, the inevitable accumulation of deposits on the surface of the electrodes would attenuate the alpha particles and prevent them from doing their job.
Alfred Hubbard. Internal-Combustion Engine Spark Plug. U.S. Patent No., 1,723,422. August 6, 1929 .
Radium Spark Plugs. Newsweek March 4, 1940 , page 53.
More Efficient Spark Plugs. Science Digest November 1941, page 94.