INSTRUMENTATION BY
R A L P H H.
MULLER
New Extremely Fast Electrometer Has Great Versatility A
NEW
CONTRIBUTION
to
the
elec
trometer a r t is afforded by the Model 640 Vibrating Capacitor Elec trometer made b y Keithley Instru ments, Inc. of 23775 Aurora Road, Cleveland, Ohio 44139. T h e 640 Elec trometer uses a unique all-glass elec trostatically driven vibrating capacitor, and can measure microvolts from ex tremely high resistance sources. I n the integrate mode, it can detect currents as small as ΙΟ - 1 7 a m p directly. I t has a sensitivity to 4 X 10~ l e coulomb. The new electrometer is also extremely fast, with a rise time of less t h a n 10 milliseconds on its millivolt range. A general view of the instrument is shown in Figure 1. T h e unit on the left is the input head with connection for the input shield lead, a four posi tion selector switch for precision shunt resistors and a zero check button. T h e unit on the right contains the o u t p u t meter and controls, counting clockwise from the upper right, a power switch reading AC, O F F , B a t t e r y , B a t t e r y Test; then a zero adjustment, fine and medium; a meter control having posi tions O F F , + , — and center zero. Next a zero check button, then a range selector for full scale values which, in microvolts, are 30, 100, 300; in milli volts, 1, 3, 10, and 30. Last, and at the lower left is the function selector, volt age and two current positions—fast and integrate. The vibrating capacitor used in the 640 Electrometer, is the Amperex X L 7900. I t is a vibrating membrane ca pacitor in an evacuated glass envelope driven by a high frequency electric field. The XL7900 consists of a metal lized glass capacitor t h a t is mechani cally oscillated a t a resonant frequency near 6 khz by a similarly constructed driver capacitor. A membrane serves as the common element for both the measuring and driving capacitors. A 1 •
