1. P. Horowitz and W. Hill,The Art of Electronics(Cambridge University, New York, 1980), p. 99.
2. Attention should be paid to the presence of metal set screws in the knob.
3. Since the connector’s receptacle will float at the input bias voltage, it should be adequately shielded to protect the operator.
4. The printed-circuit board layout is available on request from the authors.
5. We used a Keithicy model 642 electrometer. Of course, our instrument does not match the full range of applicability of that electrometer.