A 10 milliohm rheostat with 20 microohm resolution is an option for the HARS-LX Series.
For high-resolution applications, a 10 mΩ rheostat may be added for the
lowest step of the decade resistor. The rheostat is a 20 µΩ resolution
“decade”. In order to eliminate contact resistance and thermal emf, the
HARS-LX integrates the rheostat in such a way that the wiper is in the
low potential circuit. As a result, voltage and contact resistance effects are
removed by being effectively added to the input impedance of the measuring
instrument.