A lightning rod was built by Desmond after he failed to convince Claire to move her tent. The lightning rod apparently saved Claire's life when it later was struck by lightning. ("Every Man for Himself")
The implication is that Desmond foresaw the coming of a storm, and where lightning was going to strike nearby, whereupon he asked Claire to move. (In later episodes, it was confirmed that Desmond could foresee events through "flashes". Failing to convince her, he built a lightning rod. However he called his lightning rod an experiment, indicating that he was to some degree unsure about his prediction. The successful outcome confirmed that he was able to see the future, and furthermore affect its outcome via his own actions, an illustration of the theme of "fate versus free will".
Desmond later revealed that it was made to save not Claire's, but Charlie's life. ("Flashes Before Your Eyes")
HURLEY: Is that -- art?
DESMOND: Nope. Just an experiment.
HURLEY: Okay. You want some fruit salad?
DESMOND: Thanks, I'm not hungry.
[Hurley starts walking away from the shelter]
DESMOND: You might want to wait a minute.
HURLEY: Why's that?
[A sudden shower begins, then lightning strikes the lightning rod]
- The lightning rod was constructed from a wood tower, cabling from the fuselage to conduct to earth ground, and the five iron from the golf clubs, borrowed from Paulo, who at the time was driving kiwis into the ocean. The construction looked so odd that Hurley asked if it was simply artwork.