sixpence detector

This handy metal detector will greatly increase one`s chances of obtaining a piece of Christmas pudding with a six-pence in it, or if one is really lucky, a silver threepenny bit (Daddy, daddy, what`s a threepenny bit?). The principle of operation is simple and well-known. It consists basically of two oscillators, one of which is a fixed fre-quency reference oscillator, and the other an oscillator whose frequency-