Measurement photoelectric method

Different test methods produce numerically different results and comparisons should only be made of results using the same test method. Tests fall into three categories gravimetric tests, which measure ability to trap and retain dust those which measure staining power of contaminants before and after filtration and those measuring the concentration of a test aerosol either side of the filter by photoelectric methods. The tests which will be met most often are as follows. 

Measurement yields both the differences between the outer potentials and the work functions (real potentials). If two phases oc an / with a common species (index i) come into contact, at equilibrium /, (< ) = (/ ), that is at(a) - <, (/ ) = ZiFApty. These quantities are mostly measured using the vibrating condenser, thermoionic, calorimetric, and photoelectric methods. 

Figure 6.8. Schematic showing photoelectric method for measurement of splat diameter. (Reprinted with permission from Ref. 368.)...

In a variation of the photoelectric method, Fq can be determined by measuring the emission of electrons from the liquid into the vacuum. Even when Fq is negative, electrons can penetrate this barrier and be collected in the gas phase [35,36,47]. Borghesani et al. [48] used this technique and from the time evolution of the current reaching the anode for a sample of liquid Ar at 87 K found Fq to be —0.126 eV. This is in excellent agreement with the value of —0.125 eV given by Eq. (6) (see below) for this density (2.09 x 10 cm ) using the field ionization technique. 

The change in work function which accompanies adsorption on a metal surface may be determined directly by thermionic, field-emission, and photoelectric methods. Indirect methods rely on the measurement of a C.P.D. between a reference electrode and the original and covered surfaces, respectively. 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

A photoelectric method was also used by Baker and Rideal (76) for studying the adsorption of Ha, CO, and CaH on evaporated metal films of Ta, Fe, Ni, and Co. Photocurrents near the threshold were too small to be measured accurately, so the threshold frequencies were obtained by in-... 

Thermionic and photoelectric methods have been successfully employed in the measurement of the mobility of alkali metals on W surfaces. The migration of adsorbed gases like H2, O2, and CO over a metal surface may be followed in the F.E.M., and data concerning the mobility of these adsorbates are of particular interest, since they are frequently involved in surface reactions and other chemisorption phenomena. 

The photoelectric method and the method of the contact potential show only whether, and in which direction, an electron shift has occurred at the surface, owing to an interaction of the catalyst and the adsorbed molecules. By measuring the electric resistance of transparent catalyst films, however, one can find out whether complete transfer of electrons between the electron gas of the catalyst and foreign molecules has taken place (9). 

Tumerman, L. A. Photoelectric method of measuring the degree of polarization... 

The measurement of orientation angles a is experimentally more difficult than that of birefringence, and in some cases a highly sensitive photoelectric method should be used for this purpose This requirement is particularly important for the determination of characteristic orientation angles [x/g] when the measured values require careful extrapolation to both infinitely low shear rates g and zero concentration c. Figure 25 shows as an example experimental data on the dependence of orientation angle a on g for solutions of fractions of a ladder polydichlorophenylsiloxane in tetrachromethane ... 

Fig. 86. Scheme of the photoelectric method for birefringence measurements. 1 H Ne laser (X = 6300 A), 2 plate X/4 ensuring circular polarization of the beam entering polarizer 3,... 

There are several recorded determinations of the absorption curves of the aromatic amino-acids. Most of these were obtained with photographic methods of spectrophotometry which have been superceded by more accurate photoelectric methods. It will be shown that in the spectrophotometric analysis of tyrosine and tryptophan in proteins, the photometric error is magnified in the final estimate of tyrosine and tryptophan contents. This fact is inevitably bound up with the form of the equations of mixture analysis. It is therefore important that the absorption constants be measured as accurately as possible. 

Kent-Jones, D. W., and Martin, W. 1950. A photoelectric method of determining the colour of flour as affected by grade, by measurements of reflecting power. Analyst 76, 127. 

The values of (dCjdx) were measured photoelectrically, and of T a, function of x with an optical pyrometer. The results obtained by this method were comparable with those obtained by the other two methods. 

Bosworth(26) using the three photoelectric methods described on pp. 363-9 measured the migration of sodium over and into tungsten which had been out-gassed at 2200° K. and therefore retained a monolayer of oxygen. These methods were ... 

Vq values are of theoretical and practical importance. In the language of physics, Vq is identified as the bottom of the conduction band, while in chemical terms, it is called the electron affinity of the liquid. As discussed in Section 7.6, Vq reflects the subtle balance of repulsive and attractive forces acting on the electron. The more negative the value, the greater the influence of attraction, while positive Vq values point toward the decisive influence of repulsion. Vq data are compiled in Table 3. The measured data were obtained either by the photoelectric method (see Section 6.2), by electron emission from liquids, or by comparison of photoconductivity and photoemission data (see Section 6.5). Data measured by the photoelectric effect may suffer from the effect of a charged double layer at the metal liquid interface. In principle, the values obtained by the electron emission method or from the comparison of photoconductivity and photoemission thresholds should be free from this ambiguity. 

Zeeman poiarimetry Photoelectric method for measuring the displacement in the hydrogen Balmer lines by measuring modulation in the line wing between two opposite circularly polarized states. 

The final technique addressed in this chapter is the measurement of the surface work function, the energy required to remove an electron from a solid. This is one of the oldest surface characterization methods, and certainly the oldest carried out in vacuo since it was first measured by Millikan using the photoelectric effect [4]. The observation of this effect led to the proposal of the Einstein equation ... 

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

E. Photoelectric photometer method (Section 17.6). In this method the human eye is replaced by a suitable photoelectric cell the latter is employed to afford a direct measure of the light intensity, and hence of the absorption. Instruments incorporating photoelectric cells measure the light absorption and not the colour of the substance for this reason the term photoelectric colorimeters is a misnomer better names are photoelectric comparators, photometers, or, best, absorptiometers. 

The most common method of recording the absorbance of a solution was to use a micro-ammeter to measure the output of the photoelectric cell, and this method is still applied to the simplest absorptiometers. The meter is usually provided with a dual scale calibrated to read (1) percentage absorbance and (2) transmission. For quantitative measurements it is more convenient to work in terms of absorbance rather than transmittance this is emphasised by reference to the two graphs shown in Fig. 17.9. 

Methods are described for determining the extent to which original natural color is preserved in processing and subsequent storage of foods. Color differences may be evaluated indirectly in terms of some physical characteristic of the sample or extracted fraction thereof that is largely responsible for the color characteristics. For evaluation more directly in terms of what the observer actually sees, color differences are measured by reflectance spectrophotometry and photoelectric colorimetry and expressed as differences in psychophysical indexes such as luminous reflectance and chromaticity. The reflectance spectro-photometric method provides time-constant records in research investigation on foods, while photoelectric colorimeters and reflectometers may prove useful in industrial color applications. Psychophysical notation may be converted by standard methods to the colorimetrically more descriptive terms of Munsell hue, value, and chroma. Here color charts are useful for a direct evaluation of results. 

Photoelectric-Colorimetric Method. Although the recording spectrophotometer is, for food work at least, a research tool, another instrument, the Hunter multipurpose reflectometer (4), is available and may prove to be applicable to industrial quality control. (The newer Hunter color and color difference meter which eliminates considerable calculation will probably be even more directly applicable. Another make of reflection meter has recently been made available commercially that uses filters similar to those developed by Hunter and can be used to obtain a similar type of data.) This instrument is not a spectrophotometer, for it does not primarily measure the variation of any property of samples with respect to wave length, but certain colorimetric indexes are calculated from separate readings with amber, blue, and green filters, designated A, B, and G, respectively. The most useful indexes in food color work obtainable with this type of instrument have been G, which gives a... 

With the best observing conditions, it is possible for the trained observer to compete with photoelectric colorimeters for detection of small color differences in samples which can be observed simultaneously. However, the human observer cannot ordinarily make accurate color comparisons over a period of time if memory of sample color is involved. This factor and others, such as variability among observers and color blindness, make it important to control or eliminate the subjective factor in color grading. In this respect, objective methods, which make use of instruments such as spectrophotometers or carefully calibrated colorimeters with conditions of observation carefully standardized, provide the most reliable means of obtaining precise color measurements. 

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