Electrometer instruments

For most materials the volume resistivity will be greater than 10 Qm, and the measured resistance using the electrodes illustrated in Fig. 3 will be above about 10 Measurement in this range will require the use of an electrometer instrument drawing bias current less than about I0 A. Such instruments are almost invariably provided with a third terminal for connection to a guard electrode (see Fig. 2). The initial charging current for... 

Of the instmments described only some are suitable for use outside the laboratory. Where any instrument is carried into a flammable environment it should either be certified as intrinsically safe for exposure to the flammable atmosphere or isolated from the atmosphere such as by keeping it within a purged enclosure. Any probe connected to the instmment must be separately considered as a possible ignition source. Electrometers are described in [ 1531. A more general review of electrostatic instruments is given in [ 136]. 

The preferred potential-measuring instruments are potentiometers or electrometers, either of which permit measurements to be made without flow of sufficient current to polarise the electrodes during the determinations. It is also possible to use millivoltmeters if the internal resistance of the instrument is high enough to avoid any appreciable flow of current. 

There are two distinct advantages of the self-powered neutron detector (a) very little instrumentation is required—only a millivoltmeter or an electrometer, and (b) the emitter material has a much greater lifetime than boron or U235 lining (used in wide range fission chambers). 

This instrument features five detectors (Table 1.7). In the flame ionization detector, the high-speed electrometer, which ensures a very low noise level, is best suited to trace analysis and fast analysis using a capillary column. 

Figure 1. Kinetics of K efflux from Chlorella under Og stress. External K measured with cation-specific electrode (Beckman Instruments, Fullerton, model 39137) with 10" cells/ml suspended in a 10 mM Tris-Cl, ImM CaCU, pH 9 solution (10 ml, total volume). Control efflux (0—0) is linear after 20 min (denoted as 0 time here). The addition of Og ( — ) for 30 min and continuous Og (O—O), 26 fimoles/liter air flow (25 cc/min) are shown. The electrode output is amplified by an electrometer coupled to an antilog converter (12,13).

Consider the operations necessary to measure the potential difference across a metal/solution interface. Various potential-measuring instruments can be used potentiometers, electrometers, etc. All these instruments have two metallic terminals that must he connected to the two points between which the potential difference is to he measured. 

Figure 2 shows a photoelectric cell suitable for the direct determination of the photoelectric work function of a metal under the conditions of chemisorption. The electrically conducting catalyst, used as a cathode, either is inserted as a metal foil B or is evaporated from E to B. A metal layer coating the inside of the photoelectric cell serves as the anode, with a lead wire C. B can be heated electrically the leads fc are sealed into quartz and connected to an instrument (electrometer or amplifier) for... 

Figure 2.16—A miniature chromatograph. Instrument using a capillary column and a photoionisation detector. The instrument, weighing 4 kg including the carrier gas (C02), is mainly used for the analysis of volatile organic compounds (VOCs) in air pollution. The photoionisation detector, which is of limited use because of its variable sensitivity, is well suited for the analysis of hydrocarbons. The high powered UV source emits photons that have energies between 10 and 11 eV, ionising the compounds that exit the column, with the exception of the carrier gas. The ionic current generated is amplified using an electrometer and is proportional to the concentration of analytes (reproduced by permission of Photovac).

Apparatus and Procedure. Surface Isotherms. The technique for determining the n-A and AV-A curves of the lipid films has been described (6). Briefly, the Wilhelmy plate method was used to measure surface tension, from which the surface pressure was calculated (n = 7h2o—yfiim) The surface potential was measured by means of a radioactive (226Ra) air electrode and a saturated calomel electrode connected to a high impedance model 610 B Keithley electrometer (Keithley Instruments, Cleveland, Ohio). 

The Plasma Chromatograph (PC, registered trademark of the Franklin GNO Corp.) is also an instrument which does not depend on the use of magnetic fields (33). It should be well suited for the study of macroions because (a) the currents measured are well within the range of a Faraday cage-vibrating-reed electrometer detector system, and (b) the instrument operates at atmospheric pressure, thus making unnecessary the reduction in pressure from that of the electrospray chamber. 

In making an analysis of a small crystal we place it at the center of an X-ray spectrometer, provided with an ionization chamber and a quadrant electrometer. The slits of the instrument should be fairly narrow, and... 

Anon, Electrometer Measurements, 2nd Ed, Keithley Instruments, Inc., Cleveland, 1977. 

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