1-1. INTRODUCTION 1-2. The Model 720A Kelvin Varley Voltage Divider is a seven-dial primary ratio standard which meets the most exacting requirements of the standards laboratory. Absolute linearity of 0.1 ppm, temperature coefficient of linearity of 0.1 ppm/°C, and self-calibration make the 720A the most accurate instrument available for the comparison of primary and secondary voltage and resistance standards. The linearity has a small power derating coefficient of 0. 2 ppm per watt which is a- chieved by matching the resistors1 temperature coefficients close to zero. This permits operation at up to 1100 volts and 11 watts. 1-3. The Model 720A has two high input terminals. The terminal labeled INPUT 1. 0 is for general use in all Kelvin Varley applications for which direct reading, in ratio, is desired. The terminal labeled INPUT 1.1 is used for voltage measurement applications when both direct reading in voltage and over-ranging are desired. An example of this application is the measurement of standard cell voltages. Use of the 1.1 INPUT at a level of 1.1 volts in this application results in divider resolution of 0.1 microvolt and allows comparison of standard cells to within 1 microvolt. 1-4. Only thoroughly aged resistors of the highest quality are used in the Model 720A Kelvin Varley Divider. As a result, linearity change per year will not exceed one part per million unless the instrument is subjected to abuse. To assure that the divider remains within its specifications throughout its useful life, adjustments are provided for the resistors of the first three decades. The third decade adjustments normally will be used only during annual calibration. Performing the self calibration procedure sets the adjustments of the first two decades. With reasonable care these adjustments will provide instrument accuracy of 0.1 part per million at the time of adjustment.