I recently acquired an HP 3456A which has a tighter specification than the EI.  It's also a couple of decades newer and has been calibrated. So I took a deep breath and opened the EI looking for a calibration adjustment.  I found one, and tweaked it to bring the EI back into specification.  It was off by about 0.06% and now it's within its spec of 0.01%. Pretty good gear, to hold its calibration so well for about a half century!  And now it has a new lease, with its recalibration. I also monitored the EI to see what its warmup time is.  It settled very closely within about 15 minutes from a cold start.  And I mean cold; I hadn't switched it on for quite a while.  (There is a 55C oven in it, holding the reference temp constant.) Of course I don't need this unit any more but it's quaint and works great.  It's not all that much bigger than the HP but lacks the resistance measurement and a few other bells and whistles.  And the big Nixie tube readout is cool. Bob