A 100 TΩ guarded Hamon transfer standard

Guarded Hamon transfer standards are used at the National Institute of Standards and Technology (NIST) for scaling to high resistance levels. An improved design for a guarded Hamon transfer standard in the range from 1 TΩ to 100 TΩ is described. Measurements taken to select the primary and guard resistor elements are described, as well as the process used to clean the resistor elements. The selection process for resistor elements focused on choosing the smallest settling times and narrowest range of corrections from nominal value. Key aspects of this guarded Hamon transfer standard are the elimination of charge-storing materials in its connections, selection of resistors to minimize leakage currents, and the hermetic sealing of all the elements inside one enclosure.