The Keithley Model 662 Guarded dc Differential Voltmeter is an extremely precise self-contained, guarded potentiometer. It measures from 100 millivolts to 500 volts with 0,01% limit of error, and below 100 millivolts within 10 microvolts. Since the limit of error includes all stability considerations, the unit need not be periodically calibrated or manually restandardized for a full year. The 500-volt reference supply permits infinite impedance at null for measurements from 0 to 500 volts. Six dials assure that every voltage setting has at least 5-dial resolution. b. Features for convenient use include: six in-line readout dials with automatically lighted decimal points; 10 to 25-millivolt recorder output; input polarity switch; floating operation up to 500 volts off chassis ground; line frequency rejection greater than 45 db. Also, full guarding minimizes leakage problems. OPERATING MODES. The Model 662 can be used as a potentiometer or as a conventional vacuum tube voltmeter. As a potentiometer, it measures from 100 millivolts full scale to 500 volts with +/-0.01% limit of error and from 100 microvolts full scale to 100 millivolts within 10 microvolts. As a VTVM, the Model 662 measures from 1 millivolt full scale to 500 volts with an accuracy of +/- 3% of full scale. It can also measure resistances from 1 megohm to 100,000 megohms within +/- 5%.