The Model 335D Voltage Standard combines a dc voltage source and a dc voltage measurement capability in one instrument. Precision dc voltages up to 1100 volts, variable by way of the front panel readout dials, are provided at the OUTPUT terminals of the instrument. Measurement capabilities from 10 microvolts to 1000 volts are provided by a precision dc differential voltmeter. This combination allows flexible use of the instrument as a voltage standaj-4, a differential voltmeter, or as a null detector. 1-3. Protection against possible equipment failures or operator errors, which might otherwise damage expensive instruments, is incorporated. The VOLTAGE TRIP and VERNIER controls provide a msans of limiting the output voltage within the range. Should the output voltage exceed a preset limit, the OUTPUT terminals are deenergized. A current limiting circuit limits the available current to a level determined by the setting of the CURRENT LIMIT control. Therefore, it is able to provide currents up to, but not exceeding the level determined by the setting of the CURRENT LIMIT control. 14. The inner chassis and circuitry are surrounded by an isolation guard which is also isolated from the front panel and the outside cover. When properly connected, the guard bypasses any circulating ground currents which otherwise may cause error. Remote sensing of the output voltage is also used with the instrument to reduce errors caused by voltage drop in external cables. 1-5. Most of the instrument circuitry is mounted on modular plug-in cards. An extender card is provided as an accessory to aid in the maintenance and adjustment of the instrument.