The Model 515 Megohm Bridge is a Wheatstone Bridge for measuring resistors from 10⁵ ohms to 10⁻¹⁵ ohms with accuracies from 0.05% to 1.0%. It is complete, with an electrometer null detector, shielded enclosure for the unknown, and a bridge voltage supply. A unique system of switches is provided to allow corrections to be made for the slow changes in resistance of the standard high megohm resistors. This enables all values of resistance to be read with rated accuracy, directly from the bridge dials. Bridge voltages in one volt steps up to 10 volts are available from the internal supply. With external supplies, voltages as high as 1,000 volts can be used. A connector is provided so that unknown resistors can be measured outside the instrument as well as in the built-in shielded enclosure.