Original Crookes tubes and other cathode ray tubes from 1890-1900, used by Wilhelm Röntgen in his investigations of X-rays. University of Würzburg.
The Lonely Planet guidebook and the official visitor's map and guide to Würzburg both mentioned this place as a museum dedicated to Röntgen. Being a physicist, naturally I had to go and visit. I expected a small building with some displays and so on. But the building it's in is quite large. It turns out that the museum is Röntgen's original laboratory - which was in the University of Würzburg. And still is. The university has continued around Röntgen's lab, turning it into a small museum but maintaining the normal trappings of a university in the remainder of the building. So when we wandered in to look at the historical stuff, we had to walk past lecture rooms full of students, offices inhabited by professors, and so on. A bunch of students walking past looked at us strangely as if wondering what we were doing there.
Still, it was very cool and well worth it.