Contacting electrometer

Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

Viewing things from the perspective of his physical theory of contact electricity, Volta was intrigued by the apparently endless power of the battery to keep the electric fluid in motion without the mechanical actions needed to operate the classical, friction, electrostatic machine, and the electrophorus. He called his batteiy alternately the artificial electric organ, in homage to the torpedo fish that had supplied the idea, and the electromotive apparatus, alluding to the perpetual motion (his words) of the electric fluid achieved by the machine. To explain that motion Volta relied, rather than on the concepts of energy available around 1800, on his own notion of electric tension. He occasionally defined tension as the effort each point of an electrified body makes to get rid of its electricity but above all he confidently and consistently measured it with the electrometer. 

The simplest device for measuring ECC at mercury is Gouy s capillary electrometer (Eig. 10.5). Under the effect of a mercury column of height h, mercury is forced into the slightly conical capillary K. In the capillary, the mercury meniscus is in contact with electrolyte solution E. The radius of the mercury meniscus is practically equal to the capillary radius at that point. The meniscus exerts a capillary pressure Pk = directed upward which is balanced by the pressure = ftpegg of... 

If solutions of two electrolytes are brought into contact there is, generally speaking, a potential difference between them, just as there is one at the interface mercury-electrolyte in the capillary electrometer. This potential difference has been shown by Nemst to depend on the differences in the concentrations and the migration velocities of the ions. Smith uses dilute solutions containing equivalent amounts of KI and KC1 the kation is thus the same in both solutions, and the migration velocities of the I and Cl ions are nearly equal, so that, according to Nemst s theory, there should be no potential difference or double layer at the interface. These... 

Medley (M4) working in vacuum also found that mercury became positively charged relative to nylon, mica, and Alkathene6 film. He obtained the contact by bringing a column of mercury up against a film of the other material, the back of which was covered with a stationary film of mercury connected to an electrometer, which measured the charge after the contact... 

Thomson s method has been used for the investigation of adsorption layers, especially in the arrangement described by Zisman (42) [c/. also Potter (43), Frost and Hurka (44), and Rosenfeld and Hoskins (45)]. In Zisman s method the mobile electrode vibrates mechanically, causing periodic variations of the above-mentioned electrometer charge. If the electrometer is replaced by an amplifier, a signal is heard in a telephone at the output of the amplifier, which vanishes if the difference of the contact potential is compensated. 

A test system, controlled by personal computer (PC), was developed to evaluate the performance of the sensors. A schematic of this system is shown in Figure 3. The signals from the sensors were amplified by a multi-channel electrometer and acquired by a 16 bit analog to digital data acquisition board at a resolution of 0.0145 mV/bit. The test fixture provided the electrical and fluid interface to the sensor substrate. It contained channels which directed the sample, reference and calibrator solutions over the sensors. These channels combined down stream of the sensors to form the liquid junction as shown in Figure 1. Contact probes were used to make electrical connection to the substrate. Fluids were drawn through the test fixture by a peristaltic pump driven by a stepper motor and flow of the different fluids was controlled by the pinch valves. 

The solvent, and the solution, are made to flow, one in a column down the centre of a vertical tube, the other down the walls, so that there are constantly renewed surfaces of solvent and solution, of large area, in fairly close proximity. The difference in the air-liquid potential between the two liquids (which is the surface potential of the solute) causes a difference in potential between the liquids, and since the liquids are constantly renewed, current must be supplied to one liquid, and taken from the other, in order to maintain this difference. This current is large enough to be measurable by an electrometer. A circuit is therefore constructed with reversible electrodes in contact with the insulated reservoirs containing a supply of each of the liquids, an electrometer to detect the flow of current, and a potentiometer to impose any desired potentials on the liquids. The potentiometer is adjusted until the electrometer shows no flow of current then the applied potential is equal to the difference in... 

There are two practical difficulties. One is the measurement of the very small photocurrent, which may be as low as 10" amp, requiring the use of a vibrating reed electrometer or similar instrument. The second is that, for work functions above 5 eV, Vq lies in the far ultraviolet. This makes the study of some adsorptions very difficult and 6 eV is about the practical limit of such measurements. A suitable light source is the quartz mercury arc. The energy of the incident beam can be measured with a calibrated photocell, a vacuum thermopile or a radiometer. A suitable cell for adsorption studies is shown in Fig. 11. The sample being studied forms the cathode B. It can be a metal foil or a film formed by evaporation from the filament E. A wire C is fused through the glass to make contact with the... 

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