In the middle of the 20th century, the atom was all the rage. Radiation was the shiny new solution to everything while being similarly poorly understood by the general public and a great deal of those working with it.

Against this backdrop, Firestone Tire and Rubber Company decided to sprinkle some radioactive magic into spark plugs. There was some science behind the silliness, but it turns out there are a number of good reasons we’re not using nuke plugs under the hood of cars to this day.

Hot Stuff

The Firestone Polonium spark plug represented a fascinating intersection of Cold War-era nuclear optimism and automotive engineering. These weren’t your garden-variety spark plugs – they contained small amounts of polonium-210. The theory behind radioactive spark plugs was quite simple from an engineering perspective. As the radioactive polonium decayed into lead, it would release alpha particles supposed to ionize the air-fuel mixture in the combustion chamber, making an easier path for the spark to ignite and reducing the likelihood of misfires. Thus, the polonium-210 spark plugs would theoretically create a better, stronger spark and improve combustion efficiency.

These plugs hit the market sometime around 1940, though the idea dates back at least a full 11 years earlier. In 1924, Albert Hubbard applied for a patent (US 1,723,422), which was granted five years later. His patent concerned the use of radium to create an ionized path through the gas inside an engine’s cylinder to improve spark plug performance.

Firestone’s patent (US 2,254,169) came much later, granted in 1941. The company decided that polonium-210 was a more viable radioactive source. Radium was considered “too expensive and dangerous”, while uranium and thorium isotopes were found to be “ineffective.” Polonium, though, was the bee’s knees. From the patent filing:

Frequently, conditions will be so unfavorable that a spark will not occur at all, and it will be necessary to turn the engine over a number of times before a spark occurs. However, if the alpha rays of polonium are passing through the gap, a large number of extra ions are formed by each alpha ray (10,000 ions per-alpha ray) and the gap breaks down promptly after the voltage begins to rise and at a lower voltage value than that required by standard spark plugs. Thus, it might be said that polonium creates favorable conditions for gap breakdown under all circumstances. Many tests have been run which substantiate the above explanations. The most conclusive test of this type consisted in comparing the starting characteristics of many  polonium-containing spark plugs with ordinary spark plugs, all plugs having had more than a year of hard service, in several engines at -15° F. It was found that thirty per cent fewer revolutions of an engine were required for starting when the polonium plugs were used.

As per the patent, the radioactive material was incorporated into the electrodes by adding it to the nickel alloy used to produce them. This would put it in prime position to ionize the air charge in the spark gap where it mattered most.

The science seems to check out on paper, but polonium spark plugs were only on the market for a short period of time, with the last known advertisements being published sometime around 1953. If the radioactive spark plugs had serious performance benefits, one suspects they might have stuck around. However, physics tells us they may not have been that special in reality.

In particular, polonium-210 has a relatively short half-life of just 138 days. In a year, 84% of the initial polonium-210 would have already decayed. Thus, between manufacturing, shipping, purchase, and installation, it’s hard to say how much “heat” would have been left in the plugs by the time they even reached the consumer. These plugs would quickly lose their magic simply sitting on the shelf. Beyond that, there are some questions of their performance in a real working engine. Firestone’s patent claimed improved performance over time, but a more sceptical view would be that deposits left on the spark plug electrodes over time would easily block any alpha particles that would otherwise be emitted to help cause ionization.

Ultimately, while the plugs may have had some small benefit when new, any additional performance was minor enough that they never really found a market. Couple this with ugly problems around dispersal, storage, and disposal of radioactive material, and it’s perhaps quite a good thing that these plugs didn’t really catch on.

Despite the lack of market success, however, it’s still possible to find these spark plugs in the wild today. A simple search on online auction sites will turn up dozens of examples, though don’t expect them to show up glowing. The radioactive material within will long have decayed to the point where they’re not going to significantly exceed typical background radiation. Still, they’re an interesting call back to an era when radioactivity was the hottest new thing on the block.