Space Shuttle sparks

[minko]

I had been watching an old video of Jeremy Clarksons Extreme Machines which featured a close-up of the engine ignition immediately prior to lift off from the launch pad & I was wondering what is the purpose of the stream of sparks emitted directly below the main engines just before they fire up?

[plymouthflye]

It's a way of burning off any excess Hydrogen vapours that may be present as the main engines ignite. If they weren't burnt off, then it would make the thrust generated from the engines and boosters very lumpy, just when they need it to be as smooth and controlled as poss. ie. at launch.
Hope this helps.