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The 887A.AB series instruments are capable of being used as conventional voltmeters for rapid determination of voltages from 0 to 1100 volts dc and from 0.00:1 to 1100 volts ac to within ±3% of range setting; as differential voltmeters for precise measurement of dc voltages from 0 to ±1100 volts to within ±(0.0025% of input + 0.0001% of range + 5juv); as accurate ac voltmeters for measurement of ac voltages from 0.001 to 500 volts to within ±(0.05%of input + 0.0025% of range) from 30 Hz to 5 kHz, with reduced accuracy to 5 Hz and 100 kHz; and as megohm-meters for measurement of resistance from 10 megohms to 11,000 megohms with a typical accuracy of 5%. They can also be used to measure the excursions of a voltage about some nominal value. One feature that should be emphasized is that no current is drawn from the unknown source at null up to 11 volts dc. Thus the determination of the unknown potential is independent of its source resistance. Above 11 volts dc, the input resistance is an excellent 10 megohms. To minimize errors due to common mode voltages, the 887 A series is provided with extremely high leakage resistance to ground - - typically several hundred thousand megohms. Also, where ground loops errors are a problem, the battery operated mode of the 887AB eliminates these errors due to complete isolation from the power line. As additional features, the 887A series contains a polarity switch for equal convenience in measuring positive or negative dc voltages and an adjustable recorder output which makes the instrument particularly useful for monitoring the stability of almost any ac or dc voltage. Furthermore, thorough shock, vibration, humidity, and temperature testing assure years of hard use under severe environmental conditions. When used as a dc differential voltmeter, the 887A operates on the potentiometric principal. An unknown voltage is measured by comparing it to a known adjustable voltage with the aid of a null detector. An accurate standard for measurement is obtained from 11 volt dc reference supply derived from a pair of temperature-compensated zener diodes. The known adjustable reference voltage is provided by a Kelvin-Varley voltage divider with four decades of Fluke precision wirewound resistors and a high-resolution interpolating vernier that are set accurately by five voltage readout dials to give a six digit readout. In this way, the 11 volts can be precisely divided into increments smaller than 10 microvolts. The unknown voltage is then simply read from the voltage dials. For voltages between 11 and 1100 volts dc, an input attenuator divides the unknown voltage by 100 before it is measured potentiometrically. When used as an accurate ac voltmeter, the 887A operates essentially the same as for dc differential measurements. The ac input voltage is converted 10 a dc voltage and this dc voltage is measured by comparing it to a known adjustable reference voltage.