GENERAL The Model 515A Megohm Bridge is an instrument for measuring resistance from 10⁴ ohms to 10¹⁵ ohms with a limit of error from .05 to 1%. It comprises a solid-state, guarded, electrometer null detector; an ultra-stable, highly-regulated dc voltage source, and a Wheatstone bridge, 1-2. FEATURES. a. Accuracy Verification: Accuracy is traceable to the National Bureau of Standards by use of the Model 5155 resistance standards available as an optional accessory. b. Selectable Bridge Voltage: An internal voltage source spans a range from 1 to 110 volts in 1-volt steps. c. Shielded Compartment: Connection to the bridge is made using a guarded terminal in the shielded compartment which minimizes noise pickup. d. Standardize Mode: This mode can be selected for quick calibration of bridge elements to correct for slight changes in the standard high megohm resistors. e. Guard Leakage Check: A quick self check of the guard to ground resistance can be made using test jacks on the front panel (inside the measuring compartment .