The measurement of high value resistances at the NPL

Measurement of decade value resistors between 10 to the 9th and 14th power ohms is discussed. Measurements are made using a conventional Wheatstone bridge arrangement. The voltage source used to energize the bridge is made from dc-dc converters filtered such that the ripple on the output is 1 part in 10,000. It is continuously variable up to 520 V. The build up through lower resistance decades establishes a value for 2 wirewound 1 billion ohm build up boxes. This is transferred to other 1 billion ohm resistors (primarily Welwyn resistors) by substitution using a bridge with a ratio of 1. In this bridge, and in others used to determine higher value resistances, the ratio arms have low values compared to the standard and test resistors but not so low as to suffer from changes due to self heating. Decade values from 10 billion to 1000 billion ohms are measured in bridges using ratios from 10:1 to 1000:1 against the Welwyn resistors. Remaining decade values are measured in bridges using ratios of 10:1 and 100:1 against 1 to 2 1000 billion ohm resistors, previously compared with the 1 billion ohm standard.