what is it question

I want to look for something like the old slot car controllers. Basically a handgrip with a thumb operated plunger.

Basically the resistance varies with how far down you push the plunger. I'm looking for a fairly generic one because I want to open it up and swap a connection so it works backwards If I'm lucky, the design will already allow for that by just use a different pair of wires.

Anybody know what the "proper" name for one of these would be?