1-1. GENERAL. The Model 640 Vibrating Capacitor Electrometer i3 an ultra-stable, solid-state microvolt electrometer. a. As a Microvoltmeter. When used as a microvoltmeter, the Model 640 has an input resistance greater than 1016 ohms with thirteen ranges from 30 microvolts full scale to 30 volts. b. As a Picoammeter. When used with the built-in, high-megohm SHUNT RESISTORS, the instrument has twenty two ranges from 10“^ ampere full scale to 3 x 10"-* ampere. c. As a Coulombmeter. By switching an accurate guarded capacitor in the feedback loop the instrument is useful as a coulombmeter or current integrating amplifier. In the CURRENT INTEGRATE mode the instrument has ten ranges from 2 x 10? coulomb full scale to 6 x 10? coulomb, d. As an Amplifier. The analog OUTPUT permits use of the instrument as a very stable, variable gain amplifier. 1-2. FEATURES. a. Excellent Stability. A stability specified at better than 5 x 10? ampere/day is useful for mass spectrometer, resistivity, and ion chamber measurements . b. Remote Input Head. A compact Remote Input preamplifier permits convenient set up of an experiment. c. High Input Impedance. Guarding plus the use of sapphire insulation provides an input resistance greater than 10^ ohms shunted by less than 2 picofarads . d. Battery or Line Operation. A choice of battery or line operation permits complete isolation (in battery mode) from power line when required. e. Built-In Shunt'Resistors. Four high-megohm shunt resistors can be switch selected (Input Head) for shunt or feedback current measurements.