The Model 242D Resistance Measuring System provides the facility for making precision resistance measurements and comparing resistance standards . When used in conjunction with a set of Model SR 1010 Resistance Transfer Standards, the system can be used for accurately comparing different value resistance standards. For example, a 10 kilohm certified standard can be used for checking a 120 ohm resistor to an accuracy of a few ppm. The Model 242D is a major part of the equip­ment necessary for calibrating voltage dividers to highest accuracy. The system consists of Electro Scientific Industries Models 240C Kelvin Ratio Bridge RS 925D Decade Resistance Standard, and 801 D C Generator Detector. The value of the unknown resistor is read as the product of a decade reading and a multiplier reading. A deviation dial is also provided for reading the dif­ference between the actual ratio and the nominal ratio of the standard and unknown resistors in parts per million or percent. The Model 240C Kelvin Ratio Bridge is a four-terminal comparison bridge using a modification of the Kelvin double-bridge circuit. The bridge has four-terminal connections to eliminate test lead resistance in series with the unknown , It uses switches and terminals designed to minimize insulation leakage in parallel with the unknown. Front-panel controls allow lead and yoke ad­justment as part of the measurement cycle. All ratios are adjustable with narrow-range trimmers so that the bridge can be maintained with its initial accuracy indefinitely. The Model RS 925D Decade Resistance Standard provides resistance values from 10 milliohms to 1.2 megohms in 100 microhm steps, The usual zero resistance problem is eliminated by not going below 10 milliohms. The lead and contact resistances are included in the 10 milliohm resistors so that the- resistance “the bridge—'-sees" is the same as the dials read. Four-terminal connection to the bridge avoids lead and contact resistance problems between the units. The first four de­cades of the resistance standard have narrow-range trimmers on each step so that the standard can be adjusted for very high accuracy at all steps. The Model 801 D C Generator Detector provides an optimum signal source and null detector com­bination for the Model 242D System. Six Generator voltage-resistance combinations are avail­able for matching the Generator to the bridge input over a wide range of measurement values. To protect the bridge and the components being measured, no more than one watt can be supplied to the bridge with each voltage-resistance combination. The Model 801 has maximum protection from hum pickup. Provision has been made for operation by an external switch .