I have a wooden box about the size of a microwave. I have a string set up so that I can make the lid open.

I need something to reduce the amount of pull I need to open the lid. I once had a device that could be attatched to the wall of the box and
had a piston that pushed upward because of the spring inside. I used that on another project and it would be perfect for this one but I don't know what it was called or what it's intended use was.

Any help appreciated.

I think what you may be looking for is a pneumatic piston hinge

The pnumatic piston hinge is a great deal more complicated than the device I am looking for but it might work so I'll check it out,

Wha I'm looking for is just a piston that can move up and down with a spring that holds it up unless pushed on by a larger force.

I want to effectively make the lid weigh almost nothing so the string will be easier to pull.

Try a pulley system.

Is this what you're looking for?
https://www.rockler.com/lift-o-matreg-ga......d=V91040

Maybe a spring set up like a lift up coffee table, it takes very little pressure to open or close.

Hope this helps.

Mark

Have a look at this. Not anything near to what you asked about, but it is a way to make a box lid open.
https://www.frightprops.com/electric-lid-opener.html

Just a memory -once wife and I were in a restaurant -my knee hit something under the table. When knee hit 'it' the table cloth and plate rose - we called the waiter over and freaked him out. It was a loose bolt for an extension of our table.

Thanks guys. The divice I was looking for is much much simpler than those you suggest.

I actually managed to make one myself out of wood and a spring. I found but after a few tests decided the box worked better without it.