Change of the working temperature of the IEN primary group of standard resistors

To improve the temperature stability of the IEN primary group of standard resistors, their working temperature has been increased from 20/spl deg/C to 23/spl deg/C, the same value as the temperature of the laboratory. The experimental procedure adopted for this change is described. The behavior of the resistors with time, both before and after the change, has been monitored by comparison with the quantum Hall effect and is also reported. Before the change the resistors show a linear drift with perturbations due to transport. The temperature change causes the resistors to drift exponentially with mean time constant of about 0.6 years. After three years not all the standards seem to be recovering their original drift rate.