Troops have helped to reinstall the rotating optic light at the South Foreland Lighthouse in Dover.
The lighthouse warned ships of the notorious Goodwin sands
Members of the 1st Parachute Regiment, from Dover's Connaught Barracks, lifted the 200kg mechanism up hundreds of spiral stairs to the lamp room.
The National Trust says records show that a light was first displayed from the cliffs in 1367.
The light warned ships of the notorious Goodwin Sands - huge sandbanks known as "the great ship swallower".
Records also state that the lighthouse was the first, in December 1858, to show an electrically-powered light.
Forty years later, in 1898, wireless innovator Guglielmo Marconi transmitted a radio signal from the lighthouse to the East Goodwin Lightship, nine miles off shore.
National Trust spokesman Simon Ovenden said 2004 was the centenary of when the lighthouse was changed from a static or a fixed light to a rotating optic.
"To mark the centenary of the change to a rotating optic, the mechanism has been repaired," he said.
"Visitors will be able to see the optic rotating as it would have done when it was still in use."
He said people would also see how the weights, weighing a quarter of a tonne, need to be wound up by hand each hour.
After the mechanism was lifted to the lamp room on Wednesday, engineers reconnected cables and weights for the optic to rotate again using the original clockwork system - similar to that of a grandfather clock but larger.
Sgt Neil Woodhouse, of the 1st Battalion Parachute Regiment, said lifting the mechanism up the last part of the stairwell was the hardest part.
"The stairs were very tight and we had two inches either side of the machinery, with the added problem of the banister."