INTRODUCTION The Model 73IB DC Reference Standard is an ultra- stable dc power supply which, when standardized, is capable of providing a variety of precision output voltages with standard cell accuracy. The 73 IB will furnish basic dc voltage outputs of 10V, IV, ΔΕ (000 to 999 ßV), 1.018 +ΔΕ, and 1.019+ΔΕ. The desired voltage mode is selected using a front panel function switch and the value of ΔΕ is adjustable using a 10-turn linear potentiometer. The front panel ΔΕ control includes a 3 digit turn counter to indicate the selected value of ΔΕ, i.e., 000 to 999 μν. The counter is equipped with a locking lever to prevent accidental changing of a selected ΔΕ setting. A front panel guard connection is provided for use in reducing errors causedby common mode voltages. The guarded internal circuitry is isolated from the chassis and earth ground. Power to operate the 73IB can be derived from either an internal battery pack for field or portable use, or from the ac power line for bench use. A front panel meter indicates the relative charge level of the internal battery pack during portable use. The AC line requirements are 115/230V ac, 50 to 400 Hz. The 73IB is supplied with non-marring feet for bench or field use. It may also be conveniently mounted in a standard 19” equipment rack using one of the rack mounting accessory kits.