Suggest 2 stiff wires 12 inches long held 1/8 - 1/16 inches apart with  blobs of epoxy. Connect to a 741 Op Amp wired as a comparitor. If you are not familiar with these devices many books were written in the early 80's this is a pretty standard circuit.

Your local library may have electronics magazines from that vintage and these will almost certainly contain projects for this , or something very similar which you could modify.

derek.