>The most cheap system I can thing would use coils, with a piece of metal
>at the bottom of each object. Imagine two small coils (side by side)
>under each object cell, coils C and coils R.  All C coils at the same
>COLUMN are in series. All R coils at each ROW are in series.  By sending
>a scanning pulse in each ROW, the small piece of metal in each object
>will help the magnetic field to be transferred from one coil to another,
>so the COLUMNS coils will pick up that signal stronger and present at

I remember HP describing this technique for a keyboard matrix in one of their
pieces of equipment in the past. The coils were printed circuit traces. It
evidently worked quite well, but other technologies have overtaken it for cost
effectiveness in keyboards.