Two differential voltmeters: the Fluke 887A (AC/DC) and Fluke 895A (DC only).
Both are specified at 25ppm accuracy on DC ranges. The difference is the 895A has infinite input impedance at balance on all ranges, whereas the 887A presents 10M input impedance on the 100V and 1000V ranges.
The pictures demonstrate the difference. The 895A (on the right) is measuring a 10V voltage source directly. The 887A (on the left) is measuring the same voltage source, but in series with a nominal 10M resistor.
On the 10V range, they measure the same (well, 10.00035 on the 887A and 10.00000 on the 895A -- that's as close as I could match them during calibration).
On the 100V range, however, the 887A reads 4.9978 -- it's neatly divided the voltage in half with the 10M series resistor.