New Extremely Fast Electrometer Has Great Versatility - Analytical

New Extremely Fast Electrometer Has Great Versatility. RALPH H. MULLER. Anal. Chem. , 1968, 40 (1), pp 119A–120A. DOI: 10.1021/ac60257a813...
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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 •