He's completing his experiments, that's all.
4 seconds sound clip from the Frankenstein (1931) movie soundboard.
You can hear this line at 00:25:53 in the Blu-ray version of the movie.
Quote context
[...]
- You two have it all arranged, haven't you?
- You think I'm an idiot, don't you? But I'm not.
- You're quite mistaken, Baron.
- What's the matter with my son? What's he doing?
- He's completing his experiments, that's all.
- Oh, Baron, you don't understand.
- I understand perfectly well. There's another woman, and you're afraid to tell me.
- Pretty sort of experiments these must be.
[...]