I think the inductive reactance that causes the voltage drop across the field coils isn't accurately measured without a changing voltage. The same reason you can set neutral with AC from the wall but if you use DC you have to switch it in and off to read the deflection. And in my experience DC is much more accurate to set neutral. I have found especially on these old Otis machines guys like to bump neutral around to address any leveling problems. However there is more than just that going on here. Talking to a guy like Rolly in this stuff goes a long, long way. There is precious little of this kind of experience left out there.
And please correct me if my AC field drop test theory is erroneous.
