Page images
PDF
EPUB

started by heating with a flame to vaporize the metal. It will operate with as little as 3 amperes and a corresponding drop of 14 volts across the lamp, but most satisfactory results were secured with a current of about 7 amperes and a drop of about 25 volts across the lamp. It may be connected directly to a 110-volt direct-current power line through a suitable resistor. Under these conditions a practically pure cadmium spectrum of great brilliancy is obtained. There are no gallium lines to interfere between 4200 and 6400 A, and the ones that do occur are so faint as to be wholly negligible in polarimetric work.

(e) LITHIUM FLAME

The lithium red line (λ=6708A) may be obtained in satisfactory intensity by blowing lithium carbonate dust into an oxyhydrogen

[graphic]

FIGURE 15.-National Bureau of Standards apparatus for obtaining the lithium

flame.

flame. Apparatus devised for this purpose is shown in figure 15. It consists essentially of a cylindrical glass container, at the bottom of which is a small fan driven by a motor. Lithium carbonate, together with a small quantity of Ottawa sand, is placed in the apparatus. The sand, which is picked up and kept in violent motion by the fan, keeps the carbonate from packing and acts as a sand blast, grinding the carbonate finer and finer the longer it is operated. A slow stream of dried air enters at the bottom and leaves at the top, laden with lithium carbonate dust, whence it is conducted by a rubber tube to the housing around the oxyhydrogen burner.

(f) REMARKS ON PURITY OF LIGHT

In most instances absorption filters do not accomplish sufficient purification. For work even approaching precision, spectral purification needs to be used even with those sources which produce line spectra. The purity of the light used for making rotation measurements has not in the past had the attention which is due it.

If one measures a normal quartz control plate, for instance, with first one and then the other of the two D lines (A=5896 and 5890 A), which constitute the sodium doublet, a difference of about 0.08° is obtained. Using a good polariscope rotation measurements can be made with a precision of about 0.003 circular degree; while even the smaller instruments yield a precision of about 0.01°. It is obvious, therefore, that a monochromaticity approaching 1 angstrom unit is required even for ordinary work, and a considerably greater degree of purity for precision work, if the uncertainty in the rotation because of wave-length errors is to be reduced to the same order of magnitude as the experimental error involved in making the settings on the scale (matching the field) of the polariscope.

3. QUARTZ CONTROL PLATES

Quartz control plates are plates of crystalline quartz designed to be used as standards of rotation to facilitate precise saccharimetric and polarimetric measurements. They are indispensable in standardizing saccharimeter scales, and also in controlling saccharimetric and polarimetric measurements in the field, by checking the over-all accuracy of saccharimeter or polarimeter at the time the measurements are being made.

Inasmuch as the highest possible precision is frequently called for in polarimetric measurements, quartz control plates must be designed, constructed, and standardized in a manner commensurate with necessary accuracy and dependability.

(a) REQUIREMENTS AND METHODS OF TESTING

(1) CRYSTALLINE PURITY.-First among the requirements is that of purity; the plate must be of optically homogeneous quartz and contain no striae, inclusions, twinning, or other flaws, which might render the plate unreliable in service. Such flaws, even if not within the effective aperture, i. e., flaws which occur only around the edges covered by the mounting, are not permissible, since the changes in temperature may cause differential expansion and set up strains in the plate, which might make it of doubtful utility.

Plates are tested for purity by placing them between large accurately crossed nicols in a darkened room, using an intense white-light source and compensating for the rotation of the plate by means of a quartz compensating-wedge system. Flaws may best be detected by focusing the observing telescope sharply upon the plate and then rotating the plate in its own plane. Any flaw in the plate will be seen to move with the plate, and hence will be readily detected. With proper technique, this can be made an exceedingly delicate test.

(2) PLANENESS AND PARALLELISM OF THE FACES.-A second requirement is that the faces of the plate shall be both plane and parallel to a sufficient degree of precision; otherwise the plate would

be of different thicknesses at different points throughout the free opening, which would give a nonuniform field. At some points the plate, being thinner or thicker, would have less or more rotation than at other points.

Since a standard should be at least as accurate, and preferably more so, than the instrument it is to control, or check, and since the best modern saccharimeters are capable of approaching an accuracy of 0.01° S, it is necessary that quartz control plates do not differ in thickness at any point within the free aperture by more than an amount corresponding to about 0.01° S. Since a 100° plate is 1.59 mm thick, 0.01° S corresponds to a thickness of 0.000159 mm, or between one-fourth and one-third wave length in air of the light usually used in testing them.

The planeness of the faces can be tested most conveniently by observing the interference pattern formed, by reflections from the surface being tested, and from an optical flat. Either a mercury or a sodium light source is satisfactory for the purpose.

The parallelism of the faces is most easily checked by observing the Haidinger rings formed by reflections from the two surfaces of the plate.

Both of these methods as applied to quartz control plates are described in detail by Brodhun and Schönrock [38].

(3) AXIS ERROR.-It is important that the faces of the plate be accurately oriented at 90° to the crystallographic axis, i. e., when in use the light shall pass through the plate in a direction parallel to the principal or crystallographic axis of the quartz crystal. The accuracy of the construction of the plate in this respect is checked by observing the amount of displacement of the uniaxial interference figure when the plate is rotated in its own plane. This scheme was used by Gumlich [39] and later perfected by Schönrock [40], and by Brodhun and Schönrock [38]. The latter designed a special instrument for the purpose, which was built by Schmidt & Haensch and also by R. Fuess. The method is capable of a precision of a few seconds of arc, which is more than is needed, since the tolerance permitted between the crystallographic axis and the normal to the faces of the plate is 12 minutes. of arc.

(4) MOUNTING. Another factor of great importance, and one that in the past has not been given the attention it deserves, is that of the mounting. The plate should be mounted loosely in a metal frame, the axis of which forms an angle of 90° with the plate. The amount of play between the faces of the plate and the frame should be as small as possible, but the metal should exert no pressure upon the plate under any conditions.

A little play around the circumference of the plate does no harm and is desirable in checking whether the plate is free from pressure and yet has not too much play. One of the most sensitive and re

Figure 16.—A, Axis-error apparatus for testing quartz control plates, and group of mounted quartz plates and the optical flat used for testing the faces for planeness; B, Bausch & Lomb type of mount; C, Hilger type of mount; (The steel ring, C, which carries the quartz plate, G, is ground parallel and lapped down until it is just 0.00015 inch thicker than the plate. It is held in place against the flat surfaces, A and D, by the screws, F. The shoulder, B, rests in the trough of the instrument in the usual manner.); D, Physikalische-Technische Reichsanstalt type of mount (dimensions in millimeters).

[graphic][merged small][subsumed][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

liable tests is made by ear in a place free from disturbing noises. If the plate is placed close to the ear and shaken sharply in a direction parallel to the axis of the plate, no click should be heard, or at most only a very faint one. A shake parallel to the faces should produce a decided click, proving that the plate is not bound in position by pressure.

(5) MEASUREMENT OF ROTATION. Inasmuch as precision work of the highest type is frequently called for in meeting the demands for accuracy in standardizing quartz control plates, the mercury line, X=5461 A, for the reasons previously outlined, is used exclusively.

Temperature coefficient. In order to utilize this source to best advantage, the temperature coefficient, a, of quartz for this wave length (X=5461 A) and also the constant x=5892/x-5461 Where is the rotation, were determined. It was thought advisable to measure a for this wave length, although Lang [41], Sohncke [42], and Le Chatelier [43] state that it has the same value for all wave lengths.

We have

Po (1+at),

where, is the rotation at temperature t and 40 at zero. For any other temperature, t1, we have

[ocr errors]

Hence

Φι Φι

απ

The measurements [24] were made with the most improved types of apparatus. By computing a from the value of at temperatures between 4° and 50° C, the value obtained was a= =0.000144. Then between 4° and 50° C

[blocks in formation]

Conversion factor.-In measuring x-5892-5/x-5461, a large number of determinations were made, practically all of which were concordant. However, in order to eliminate the personal equation and avoid, as far as possible, errors due to the character of the sodium source, the value of is computed from the measurements of five plates whose sodium values have been determined at the PhysikalischTechnische Reichsanstalt. The mean of these values and those of this Bureau was taken as the rotation for λ=5892.5. The rotations were measured in part with a sensitive-strip polarizing system. The greater number, however, were made with an exceptionally good Lippich system. The average value obtained was

[blocks in formation]

Thus any quartz rotation for the wave length 5892.5 may be obtained by measuring the rotation for the wave length 5461A and multiplying it by the constant 0.85085. By this method the errors due to the character of the sodium source of light are eliminated, and the measurements of one observer may be readily compared with those of another. National Bureau of Standards certificates show the rotation in circular degrees at 20° C for wave lengths λ=5461 and X=5892.5 A. The latter is the so-called optical center of gravity of the two sodium D1 and D2.

« PreviousContinue »