A Universal Potentiometer for the Range from One Nanovolt to Ten Volts

Recent interest in accurate dc measurements down to nanovolt levels has prompted the development of a wide voltage range potentiometer of a new type with 7 dial resolution and extremely high accuracy. The potentiometer design is almost perfectly "neutral." All switch contacts and resistors of the main potentiometer operate at high voltage levels, and contribute thermal EMFs which are attenuated by a factor of ten thousand times (to less than 0.1 nanovolt levels) on the nanovolt range. The device serves as an "autocertified" voltage potentiometer standard. It is certified traceable to National Bureau of Standards units using previously published " ratiometric" techniques and is capable of direct reading accuracy without correction from 2 ppm on the 10-volt range to 10 ppm of setting ±1 nanovolt on the 0.001-volt range. The Thevenin equivalent source resistance of the instrument is approximately 2.5 ohms on the 0.001-volt range, reducing Johnson noise levels to less than 1 nanovolt with practical null detector smoothing times. The instrument is designed so that it can be used both as a potentiometer and as a 7-dial Kelvin-Varley divider. The control portion of the instrument is separately packaged and can convert dividers now in use to this new type of universal potentiometer standard. This paper includes a complete theoretical study of the design characteristics of this state of the art potentiometer, and an analysis of all sources of error in the completed instrument.