Ceiling – Finding ceiling joists in an old apartment

I'm struggling to find the ceiling joists from which to hang lights in an old apartment.

The building is in central Copenhagen and was built in the 1850s, so I can't assume that the joists are spaced 16" or 24" apart. Furthermore, there's quite a large, decorative cornice so nor can I simply measure out from where the wall meets the ceiling.

And to exacerbate the situation, I don't even know which way the joists run. I would expect them to span the shortest distance but I have a suspicion that my apartment and the apartment through the wall were originally a single dwelling which would make the shortest span originally the longest span. I could compare with the wooden floorboards but these are brand new and I'm not convinced that they've been laid directly on the joists instead of underlying, older floorboards (different floors are at slightly different heights but all the floorboards run in the same direction).

I tried to use magnets to locate nails or screws which would fasten the ceiling plaster/gypsum to the joists. But what I found was puzzling: the magnets stick to the ceiling in a continuous line across the short span, from wall to wall. And this repeats 3 times 6-8" out from the front wall, then 24.5" inches across the rest of the ceiling.

So I assumed that this was a form of fastening to a joist and drilled a small hole immediately to the side of the very narrow magnetic line. The drill went right through without finding wood.

I put a U-shaped piece of wire into the hole, intending to turn it until it hit a joist (and the side of the 'U' below the ceiling would point to the joist) and a lot of grainy, grey dust poured out of the hole. I tried it again and more dust poured out. In fact, each time I try to insert the wire, a continuous stream of the grey dust pours out.

Also, the building was renovated immediately before we moved in so there's only one indication of where a light was previously hung, and it's in an odd location where we don't want a light.

We've asked a couple of neighbours and both are using only floor-standing lamps because of the lack of clues on the ceiling.

Does anyone have any recommendations? All I can think of doing now is drilling more holes until I hit wood, but without knowing how far apart or how wide are the joists, or even which way they run, I could end up with more holes than plaster/gypsum on the ceiling.