Special shielded resistor for high-voltage d-c measurements

Abstract

A new d esign for an acc urate high-voltage d-c standard resistor has been dc \·ised. It is m a de up of a large number of individua ll y s hielded, one-m ego hm wire-wound resisto rs co nn ected in series a nd arran ged to form a vertical helix between a ground plate a nd a high-voltage electrode. The indi vidua l shields co mplete ly e nclose each one-megohm resistor and prevent formation of co rona at t he s urface of t he resistance co il no matter how hi g h the potential of t he hi eld is above g round. The ve rt ical helical configu ration with a large " hat," or high-voltage electrode, on top serves to prevent conce nt ration of electri c fi eld a nd corona formation at t he hi g h-potentia l end of t he resistor. A 200-megohm uni t, constructed during 1955 a nd t ried out up to 100 kv in 1956, indicated t he des ign to be free of corona errors but, for t he part icular one-megohm resistors used, the variation with tempera-t ure \Va q ui te la rge (0.01 percent per 0G). A 100 m ego hm unit usin g low-temperature coefficie nt res istors has been recently co nstru cted a nd tested up to 100 k v. An experim e nta l method of c heckin g for corona or leakage errors at hi gh values of voltage was deve loped. It consisted of accu rately comparin g the curre nt "in" at t he hi g h voltage end of the resistor with t he cLllTent "ou t" at t he ground end for se veral different values of app lied voltage. These measurements together with others performed to check temperature a nd lea kage errors indicated t hat the valu e of res istance for t he 100-megohm unit rema ins co nstant to within abo ut 10 ppm for voltagcs up to 50 kv under ordina r y laboratory conditions. At 100 kv t he maximum error (ca used by heating) was estimated to be a bout 40 ppm.