Just a suggestion. Never tried it before. But I work with Aluminum and we have some device that the quality control guys in the shipping and recieving office use to measure the density ( I think)of the metal we get.It's like a key chain and you just press it on the metal surface and it gives out a reading. Maybe there is some type of gauge made that measures brass in this type of fashion.I don't know but it's an idea that may work better than the paper clip. I don't know what this device is called, but I'll find out Monday.