The oldest building in Brisbane

I was travelling along Wickham St in Brisbane near the University of Queensland, when I came across this odd looking building. I was unsure what it was with its copper ball on the top assuming it to be some sort of university buidling gone wrong. It looked as if it was some sort of lookout but it was miles away from the river. Then I saw the sign nearby. It was an old windmill that had it's function altered over time.

The Windmill on Wickham Terrace was constructed by convicts in 1828 and is today the oldest building still standing in Queensland.Because of a fundamental design flaw it never worked as a windmill, and 25 convicts at a time were put to work turning a treadmill which became known as the "tower of torture". For special punishments, 16 convicts were kept on it for 14 hours straight.

Over a long period the building has been subjected to various uses including a treadmill, a signal station, a time ball, fire brigade and as a building for radio and television research.

I'm still no closer to finding out what the copper ball was for.... perhaps some sort of Tesla device?