The plug is wired with DC on the sleeve and ground on the ring. The tip is unconnected, presumably for product-liability reasons: once the plug is within its metal barrel (seen at left in the photo), only the tip protrudes. If it touches metal etc, there can be no arcing since it's no connected to anything, while the hot segment of the plug is deep with the outer barrel.

  The barrel is connected ground through the small resistor visible in the picture, presumably to bleed away any static charges which a person may be carrying when they touch the plug. The most important function of the barrel appears to be supporting and steadying the plug when it's inserted into the computer.

  The plug still works without the barrel, and indeed, I replaced this one with a miniature TRS audio plug, and the iBook charges and works normally. The plug alone, though, it really too fragile for day-to-day use. A solid repair needs the same type of plug, with the outer supporting barrel.

  For a yo-yo adapter, I'd also add some shrink-wrap. Even then, the wire in these is of such poor quality that the strands are practically guaranteed to break again abovce the shrink-wrap. But for at least as long as there was enough wire left to cut off the bad section and attach a new plug, it could still be repaired and used.