At a RenFez in Kansas City, I recently saw a fantastic and hilarious routine (Square circle of all things) with wood cutout words produced. I asked the performer if it was his, and he told he it was entirely (not the SC of course) his routine-so it will be a joyous memory. I will never use his routine.
This question refers more to sleights and routines rather than devices.
It's the old catch 22, if you show it to someone-they may figure it out and do it themselves-it is no longer yours. If no one (in magicdom) sees you do it, they will never know you thought of it first.

How long do you think a move/sleight/routine should be the individual's? When do you think it become public domain? Before you answer, how many people are paying Selbit's estate for sawing a woman.....

How many variations/permutations does an effect undergo before it no longer is the originator's?

Do you think there are some magicians, possibly bred by our information explosion, who are more interested in making a living from magicians, than from peformances? If so, is this beneficial or detrimental to the art?
David

There is another option on routines and ideas.

ASK the inventor. Unless you are seeking to publish a variation or derivative work, it's usually not too difficult to get a 'yes' answer for performing a thing.

It gets a bit more tricky when publishing stuff. There may be a ton of other unpublished or private things involved in the trick you like, and so there may be issues about its publication. Even if there are not strings of living people's secrets involved, it may be something they feel attached to.

If all you want to do is perform something, try asking. Perhaps if you have something of yours to offer, the reluctant might be more inclined to accept your request and offer.

We have journals to manage the publications and credit stuff. Josh Jay in Magic Magazine, Richard Kaufman, JonR at Genii. And also other magazines are out there. If credit is what you want... let THEM know.

When did the Special Theory of Relativity enter public domain?

Best, PSC