Today’s invention is handcuffs which automatically open after a given time period.
These would be cheaply made and come with a clockwork mechanism embedded which allowed the owner of the cuffs to specify how long they would stay locked.
Cuffs like these might be useful if a police force needed to restrain rowdy individuals for a while, without actually imprisoning them, or to impose house arrest on someone for a short period (perhaps for their own safety).
The clockwork could be wound using the cuffs’ own key which would then be stored in a small box with a push-to-lock door attached to one cuff.
After a set time, the box would spring open.