spark plug

Noun

spark plug (plural spark plugs)

1. The part of some internal combustion engines which forms a high-voltage electric spark which ignites the fuel-air mixture to begin the power stroke. (Petrol engines have spark plugs. Diesel engines do not have them because the heat from their high compression ratios supplies the ignition.)
   • In the 21st century, it is not unusual for a set of spark plugs to last a decade or longer; gone are the days when changing the plugs annually was common.
   • With all engines using spark plugs it is comparatively easy to ascertain whether the ignition system is working properly.1919 January, Practical hints on running a gas engine, United States Department of Agriculture
2. (figuratively) Someone who is a driving force in new endeavours.
   • Helen is a real spark plug. We're lucky she's on the team.
   • Steve Pincus, a Madison-area farmer who sells produce to L'Etoile, says admiringly of Odessa: "She's a spark plug. She's out there speaking out about sustainable agriculture and local food."2003, Growing for Market: News and Ideas for Market Gardeners

   Synonyms:

   straw that stirs the drink, catalyst, instigator, mover and shaker, stimulus

3. A rod (for cylindrical secondaries) or sphere (for spherical secondaries) of fissile material at the center of the secondary of a multistage thermonuclear weapon, which undergoes nuclear fission when compressed to supercriticality by the imploding secondary, helping to raise the secondary to the pressures and temperatures necessary to ignite nuclear fusion in the secondary's fusion fuel.