The 883AB is a high accuracy AC/DC Differential Voltmeter. Differential
voltmeters measure voltage by bucking the unknown voltage against an
internal reference voltage using a Kelvin-Varley Divider. When at null
it presents an extremely high input impedance to even high impedance
sources. This reduces measurement errors due to loading. TVM mode
configures the panel meter to indicate polarity and determine the
correct voltage range. Null mode configures the panel meter to monitor
the kelvin-varley divider output with four increasingly sensitive null
ranges. Basic dc voltage accuracy is 0.01% of indicated value + 5 MicroV.
Resolution is 1 MicroV. The unit can be battery or line-powered.