Determination of /spl Omega//sub VTT/ using the quantum Hall resistance

This paper describes the maintenance and traceability of resistance in VTT. The bank of the primary standard resistors consists of four Thomas-type 1 /spl Omega/ resistors in an oil bath (T=20/spl deg/C), a Thinsley 100 /spl Omega/ resistor (100 /spl Omega//sub Thinsley/) in an air bath (T=25/spl deg/C or 31/spl deg/C) and a ESI 10 k/spl Omega/ resistor (10 k/spl Omega//sub ESI/) located in an air bath (T=23/spl deg/C). Both 100 /spl Omega//sub Thinsley/ and 10 k/spl Omega//sub ESI/ can be calibrated directly against the quantum Hall resistance (QHR) using a low-frequency cryogenic ac resistance bridge based on the Cryogenic Current Comparator (CCC). The cryogenic ac bridge is also used to trace 1 /spl Omega/ standard resistors from 100 /spl Omega/. All these standard resistors can be traced to the QHR within 10 ppb. The calibrations are mainly performed at 1 Hz but the frequency dependence of the resistors are first determined in the frequency range from 0.1 Hz to 1 Hz.