A magnetic core portion 16, a residual base material portion 42, and a connecting portion 43 are formed by pressing from a base material 40. The dial comprises an optically conducting basic body whose outside is shaped as a two-stage cone. Also included in the circuit is a voltage shifter for shifting the voltage signal to a value other than 0 volts, such as an adder or subtractor. The first maximum perimeter is one of the same as the second maximum perimeter and disposed inside of the second maximum perimeter. The light emitter is configured to emit light on the upper surface of the sample.