There is this metal thing shaped as a 'U'; you hit it against something and put it against your ear to hear this sound. I'm not sure what it's called but I need it to tune the A string on my violin.
If it's U shaped it won't work. It needs to be Y shaped, so the handle can be held without damping the tines. Hold the handle onto something like the violin body, and the sound will be amplified. It's usually called a tuning fork, although a couple of my students call it a pitchfork, which, actually, it could be...
I guess it is also called a diapason.