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It consists in principle of two parallel telescopes having a cominon eyepiece, and is enclosed in a cast housing. The device for measurably varying the intensity of the light is shown in the drawing.

The two diaphragm leaves, with 90° openings, are movable toward or away from each other by equal amounts by means of the micrometer screws. These screws are turned by means of the drum upon which scales are engraved. When the drum is set at the scale-reading 100, the diaphragm is full open; and at 0 (a complete drum revolution) it is completely closed. One scale reads directly in values of transmission and the other in extinction (-log T). The graduations are read from a common index line engraved on a small glass plate. Since both telescopes are provided with photometric devices, the substitu

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tion method may be used. The instrument may be mounted either in a vertical or a horizontal position. Supports, absorption cells, and illuminating lamps with appropriate housing, including a mercuryvapor lamp, are available. The Zeiss pamphlet [21] describes these in detail.

Four different kinds of color filters designated S, K, L, and Hg, respectively, are supplied for use with the photometer, the Hg series being used with the mercury arc, and the others with incandescent tungsten or other white-light source.

There are nine filters in the S series characterized by high optical density and narrowest transmitted spectral band of the three series. The seven filters of the K series transmit relatively wider bands. and more light than the S series, while the three L filters have high transmission and are used for faint luminosities. Landt [22] states that the Pulfrich photometer with a mercury arc as light source, and with the appropriate spectral filters, may be recommended for purposes of color measurement in the sugar industry. doubt one of the 560-mu filters described under (c) p. 314 could be used to advantage with this instrument.

FIGURE 68.-Zeiss - Pulfrich photometer (schematic section). 01, O2, Object lenses; M1, M2, photo

eter drums with scales; L1, L2, lenses; P1, P2, prisms; P3, biprism; Ok, ocular lens.

5. PHOTOELECTRIC PHOTOMETERS

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Photoelectric cells of various kinds have become available at moderate cost within the past few years, and their use in the construction of colorimetric apparatus has become widespread. This is true particularly in the field of analytical chemistry, a review of which, with an extensive bibliography, has been published by Müller [23]. The advantages and limitations of photoelectric cells in colorimetry have been discussed by Gibson [24].

Among the advantages of photoelectric over visual methods may be named: Greater objectivity of measurement; relief from eyestrain and fatigue; and, under good conditions, greater speed. The precision and

reliability of results depend upon the design and construction of the apparatus. Specific literature references to the measurement of sugar color with certain photoelectric instruments are given in references [25 to 33] at the end of the present chapter. Some of the instruments there cited had been designed primarily for sugar colorimetry, while others evidently were intended for purposes of analytical chemistry. For the measurement of transmittancy of sugar solutions over a great range, as, for example, in comparing the decolorizing efficiency of carbons, a flexibility is demanded that is not usually present in analytical apparatus. This may be attained by providing for (1) the accommodation of a variety of thicknesses of absorbing layer up to 20 cm and (2) for the employment of monochromatic light. The second of these considerations is more important in photoelectric than in visual photom

etry over wide transmission ranges, because when a photocell is illuminated by a spectral band that is too wide, it responds to all the impinging light, whereas the eye, being unable to integrate over broad spectral band, responds to the effective wave length.

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Discussion of the numerous optical and electrical arrangements that have been employed in the construction of photoelectric photometers

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cannot be attempted FIGURE 69.-Objective sugar colorimeter of Sandera. here. This is to be

found in the references cited. The instruments described in the following pages have been used particularly for the measurement of sugar color and will serve to illustrate some principles of design and construction.

(a) SANDERA OBJECTIVE SUGAR COLORIMETER [25]

The apparatus devised by Sandera is illustrated in figure 69. Light from the incandescent filament inclosed at O passes through the diaphragm and lens at e to form a practically parallel beam, which is transmitted by the solvent in cell, a, or solution in cell, b. These cells rest upon a movable carrier, whereby the cells may be interchanged as desired. The transmitted beam enters the window of the phototube, A (a potassium cell) and strikes the sensitive surface, K. The voltage is supplied from a battery. The photoelectric current is amplified by means of the vacuum tube, while the millivoltmeter in the plate circuit is the indicating instrument. By means of the variable resistance, R3, the sensitivity of the millivoltmeter may be varied so that dark- and light-colored solutions may be measured equally well.

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Color filters are introduced at f. The voltage of the light source may be regulated either by means of an iron resistance or manually by means of a rheostat and voltmeter.

The null, or "dark," current on the millivoltmeter, always present in this apparatus, is designated as eo.. To obtain the transmittancy of a solution the cell containing the solvent is placed in the light beam and the reading, e, is noted. The cell containing the solution is next exposed and the reading, e2, noted. Then -log t is calculated.

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(b) HIRSCHMÜLLER APPARATUS

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This instrument was first described briefly by Landt and Hirschmüller [29] and later in greater detail by Hirschmüller [34] in a Beer's

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FIGURE 70. Hirschmüller photoelectric colorimeter.

1, Mercury arc; 2, plug connector; 3, reactance; 4, spectral filter; 5, filter holder; 6, three-part condenser; 7, fixed aperture; 8, lens; 9, variable aperture; 10, tube; 11, diaphragms; 12 and 121, cells for solution and solvent; 13, achromat; 14, photocell; and 15, iris diaphragm.

law study of beet sugars. Details of construction of the photometer and of the Multiflex galvanometer are shown diagrammatically in figures 70 and 71.

As shown in figure 70, the light source at 1 is a high-pressure mercury arc operating on a 180- to 250-volt alternating current. An inductance, 3, limits the current to 1.3 amperes. Fifteen minutes after starting the lamp it burns with full intensity and with little change in voltage. The emitted light consists of a weak, continuous spectrum, which is disregarded, and a number of intense lines of the mercury spectrum, the ultraviolet portions of which are absorbed by the glass of the lamp. The wanted visible lines are isolated by means of colored-glass filters analogous to those described under (e), p. 324. The filters, held in the revolving holder, 5, are readily changeable. The absorption cells, up to 5 cm in length, at 12 and 12', rest on a revolving platform controlled by a knob above the housing and are thus interchangeable. The light beam, defined by properly arranged lenses and diaphragms, reaches the selenium barrier-layer photocell with minimum divergence.

The response of the photocell to various light intensities is measured by means of the Multiflex galvanometer of Lange [35], illustrated diagrammatically in figure 71. The distinguishing feature of this

galvanometer is that the image of the straight lamp filament is reflected by the galvanometer mirror and by fixed strip mirrors over the paths represented by the broken lines to the 20-cm scale of the ground-glass window at 5. The working distance of 1 meter is thus attained in a relatively small housing. Using a pair of matched absorption cells in the manner already described, the transmittancy of a solution, T=deflection solution/de

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PHOTOTUBE PHOTOMETER

The apparatus described below is a modification of the photoelectric colorimeter of Withrow, Shrewsbury, and Kraybill [38] and is used at the National Bureau of Standards for the measurement of transmittancy in sugar solutions. Either a mercury arc with appropriate filters. or a Mazda lamp with a 560-mμ filter, may be used interchangeably as a light source. The optical bench accommodates absorption layers up to 20 cm in thickness and the measuring bridge is simplified. The phototubes may be operated from either 115-volt alternating- or direct-cur

FIGURE 71.-Multiflex galvanometer of Lange.

rent, the latter being the 1, Lamp; 2, prism; 3, galvanometer mirror; 4, lens; 5, scale; and more satisfactory. The

6, adjusting knob.

apparatus is illustrated schematically in figure 72, and by figure 73.

The light source is at S. The mercury arc used is a vertical quartz Uviarc, 22.5 cm over-all in length and 1.8 cm in diameter, giving an arc about 7.5 cm in length. The lamp is fixed to the door of the lamp housing, which is hinged so as to open downward. By lowering the door, the mercury flows to the tungsten anode and the arc is struck when the door is replaced. At either side, and close to the lamp, are metal diaphragms with 12.5-mm openings. Fins on the diaphragms aid in disseminating heat. The lamp housing is ventilated by openings at the back and a cover with a chimney may be used. On either side of the source, the instrument is symmetrical except for the absorption-cell carrier, which is placed on the right side of the instru

ment.

On either side of the light source are lenses at L1 placed so that the source is in their focal planes, and at L2 are long-focus lenses, placed so that an image of the Mazda filament is focused on the surface of the cathode of the phototube. The diaphragms covering these lenses have 12.5-cm openings. Between each pair of lenses is a 4-cm absorption cell containing a solution of CuSO4.5H2O, 45 g per liter, to remove infrared and visible red from the mercury spectrum. Shorter cells may be used and would require proportionately greater concentration of the salt. In the large compartment at the right is a sliding brass plate with two shallow grooves, as indicated, in which are placed cylindrical absorption cells, one for the solution and the other for the solvent. Either of these may be brought into the light beam, as

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FIGURE 72.-Phototube photometer used at National Bureau of Standards for sugar solutions.

desired, by means of the rod, R. The glass spectral filters are mounted in blocks, F, in which are corresponding openings admitting the light to the phototube. The latter are cesium oxide vacuum cells supplied by the Continental Electric Co. When covered, the metal compartment serves as light and electrical shielding. The whole is built upon a bed plate of 0.5-inch pressed steel, which in turn rests on three steel bars, each having a leveling screw at one end. The cover is sheet aluminum connected to ground.

The phototube may be operated from the 110-volt supply, direct or alternating current, the cathodes or light-sensitive surfaces being connected to the negative terminal of the direct current supply or to ground through the 25,000-ohm resistors. The anodes or electron collectors of the phototubes are connected through the ratio arms of the bridge to the positive terminal of the voltage supply.

The arm at the right consists of two wire-wound radio rheostats in series, the nominal resistances of which are 10,000 and 1,000 ohms, respectively. These are for coarse and fine adjustment in calibration.

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