Showing papers in "Journal of the Optical Society of America in 1925"

Analysis of Bi-Metal Thermostats

Abstract: The following investigation contains a general theory of bending of a bi-metal strip submitted to a uniform heating. This theory is applied in analysis of operation of a bi-metal strip thermostat. The equations are obtained for calculating the temperature of buckling, the complete travel during buckling, and the temperature of buckling in a backward direction.. By using these equations the dimensions of the thermostat for a given temperature of operation and a given complete range of temperature can be calculated. The results obtained are based on certain ideal conditions. For example, it was assumed that the differ­ ence in the coefficients of expansion remained constant during heating, that the friction at the supports could be neglected and that the width

A modified Hartmann test based on interference

Abstract: A method of determining the spherical aberration of a lens is given which requires relatively simple apparatus and which gives directly the retardation of different portions of the wave-front referred to any point along the axis andintermsof the wave length of the light employed. Data are given from which a convenient self-contained apparatus can be built to serve as an attachment on an optical bench of usual design. The method is based upon interference phenomena and the bench does not need to be provided with scale or micrometric measuring devices. With the attachment built as suggested, in addition to its dimensions, the measuring of the photographic plate gives all the data required and the one necessary exposure provides a permanent and complete record of the test.

Spectral Energy Characteristics of the Mercury Vapor Lamp

Abstract: By means of a large aperture quartz spectroradiometer the spectral energy distribution of a special variable-length mercury lamp was measured between 14,000 A and 2300 A under widely varying conditions. The effects of current, voltage and ventilation on the radiation at fifteen important maxima were measured, including three maxima in the infrared, four in the visible, and eight in the ultraviolet. The composition of each maximum is given, and curves are plotted showing the degree of resolution used; the variation in intensity of each maximum with voltage gradient at constant current; the variation of intensity of the strong ultraviolet maximum near 3660 A with voltage for five current values from two to four amperes; the stationary characteristic curves of a high pressure arc; two typical cases of spectral energy distribution at constant power; and the variation with current of the intensity of each maximum at constant power.It was found that for constant current the energy in each maximum except those in the infrared increased linearly with the voltage after a certain minimum (near 8 volts for 2.5 amperes) was reached. The pressure was measured for each stationary state used, the extreme values being 20 mm of mercury and two atmospheres. Where power input was constant the energy radiated in each maximum increased rapidly with decreasing current. It was found that the most satisfactory condition for running an arc at pressures under two atmospheres, at least, was to have the pressure as high as possible, so that for a given power input the voltage was high and the current low. This condition gives the greatest efficiency and the least change in energy distribution with total energy variation. The relation of pressure to the other variables and the effects at higher pressures are being investigated further; also an absolute determination is being made of the energy available in each wave-length for photochemical purposes.

Piezo-Electric Standards of High Frequency

Abstract: It is shown that the qualities sought in a frequency standard are present to a high degree in properly mounted plates or rods of quartz, or steel rods excited by quartz plates, making use of the piezo-electric reactions of the quartz upon a resonant high-frequency circuit. The transverse effect (vibrations perpendicular to the impressed electric field) is chiefly utilized in the construction of piezo-electric resonators. Any given resonator may be excited to resonant vibration at its fundamental frequency or at any one of several overtone frequencies. The logarithmic decrement is very small, of the order of 0.001 or less. Hence a tube generator circuit of variable frequency may be tuned to a natural frequency of the resonator with very great precision.The reactions of the resonator upon the circuit may be expressed in terms of characteristic changes in capacity and resistance. Curves illustrating this are shown, and it is seen that at resonance the resonator functions as a pure resistance.In the construction of resonators, a quartz plate cut in the form of a comparatively narrow rod is placed loosely between two brass side-plates on an insulating base. Effects of changing temperature and of spacing of side-plates are discussed. For frequencies below 50 kilocycles per second steel rods are employed, excited to longitudinal vibration by having small quartz plates cemented to them at the center. By this construction frequencies as low as 3000 cycles have been reached. The upper limit in frequency, using very short quartz rods, is about three million cycles. Over this range the frequency of a vacuum tube generator may be synchronized with the resonator with a precision of one hundredth percent or better. Some of the various electric circuits that can be employed are briefly described.The application of the resonator as a frequency stabilizer, and also as a device for generating electric oscillations, is explained. For these purposes either the transverse effect or the longitudinal effect (vibrations parallel to the electric field) may be used.

Photographic Sensitometry With Fluorescent Oils

Abstract: The changes in sensitivity and contrast obtained when Cramer Contrast Process, Seed 23, and Wellington Super-Xtreme plates are coated with any one of five different fluorescent oils were studied in the region 2200-3800 A, for the purpose of improving the precision of photographic photometry in the region 2000-2400 A, and to investigate the feasibility of extending this method into the extreme ultraviolet. Characteristic curves are given for various plate-oil combinations, and some of the changes in sensitivity and contrast found are tabulated. The effect of thickness of coating was found to be great with most oils; certain oils gave the greatest improvement in sensitivity with thick coatings, and others with thin ones. The greatest sensitivity increase was found with a clear paraffin oil and contrast plates; the greatest contrast was found with a light lubricating oil and the same plates. A 400-fold gain in exposure time by oiling was found in certain cases. Some oils were found to increase the density produced by a certain intensity if the intensity exceeded a certain value, and to decrease it otherwise. This effect varies with wave length, so that it was found possible to decrease the sensitivity at longer wave lengths and increase it at shorter wave lengths, thus making the sensitivity curve of the plate more uniform throughout the spectrum. Although the chief effect with most oils is below 2400 A, certain varieties increase the contrast even up to 3800 A, although the sensitivity was generally decreased above 2500A. Oiled plates, when the oil coating is uniform, were found to be perfectly applicable to photographic photometry, accuracy to one per cent being readily attainable even at the shortest wave lengths studied. Sections of the “characteristic surface” in which the reciprocity law holds to within one per cent were found for several wave lengths, including 2200A.

Multiple Electron Transitions and Primed Spectral Terms

Abstract: Excitation of the lines of Mg+in low-voltage, low-current discharge. The stronger lines of Mg+ appear at potentials equal to the ionization potential of Mg plus the excitation voltages predicted from their classifications. This proves that a single impacting electron can ionize an atom and raise a valence electron of the ion to a higher orbit.Excitation of the pp′ lines of Mg and Cd in low-voltage, low-current discharge. Using a quartz spectrograph to analyze light from the force-free space between grid and plate, it was found that the Mg pp′ group (2776 to 2783 A) appears at current densities less than 0.2 milliampere per square centimeter. Since these lines are produced by electron transitions from an initial state in which there are two electrons in virtual orbits, this observation shows that the two electrons are moved to these orbits as a result of a single collision; lines which could be produced only by successive impact are entirely absent at much larger current densities. The pp′ group of Cd (2239 to 2329 A) appears at its quantum voltage, but is too faint to be detected at current densities which exclude the possibility of successive excitation.Several lines of Cd, In, and Tl are classified by the use of primed terms.Extension of the subordinate series of Cd: spectra of Cd below the ionization potential. The low-voltage arc in Cd yielded a number of new lines of the singlet and triplet subordinate series. In exposures taken below the ionization potential the lines of Cd are excited exactly at the voltage intervals predictable from the classification of the spectrum.

Precision Densitometers for Photographic Photometry

Abstract: After discussion of the advantages of physical densitometers and microphotometers over the older types using the eye method, the merits and disadvantages of the more important recent designs are summarized. The conclusion is reached that the thermoelectric densitometer is most accurate for direct-reading work, while the photoelectric cell is best adapted for the null method. The disadvantages of the thalofide cell and the selenium cell seem to outweigh their only advantage of large currents.A simplified standard type of direct-reading densitometer is described, with vertical optical system, in which accuracy equal to the best claimed for the photoelectric null method, one-tenth of one per cent, was attained. The sensitivity was such that 1000 mm deflections on an easily read scale were obtained through a clear plate with a slit.05 mm wide, using only 20 watts in a lamp rated at over 70 watts. A single easily constructed thermocouple was used with a low resistance d’Arsonval galvanometer. Curves are given showing the gradual improvement of deflection steadiness, from variations of two per cent to almost complete steadiness, as various sources of disturbance were eliminated.

The Influence of Temperature on the Photoelectric Effect of the Alkali Metals

Abstract: Special cells having a hollow central cathode were immersed in liquid air for an extended period to insure that any gases, if present, were condensed on the outer alkali metal coated walls. The temperature of the cathode was controlled by a stream of evaporating liquid air, whereby all temperatures between +20 and −180°C could be attained and held constant and be measured. In these cells the variation of photoelectric current with temperature in sodium, potassium, and rubidium is continuous, without abrupt changes. The effect is relatively small for sodium, showing hardly at all for blue light or white light, but clearly for yellow light. The behavior of rubidium is similar to that previously reported for potassium.In a second form of cell, potassium was collected in a deep pool. By slowly cooling the metal from the molten condition, smooth crystalline surfaces were obtained. With these annealed potassium surfaces, the variation of photoelectric current with temperature is represented by curves varying systemmatically in shape with the color of the light, and the effect is far greater than previously reported, amounting, for yellow light, to a variation of 10 to 15 times between room and liquid air temperature. When the surface is roughened curves of the previously reported type are obtained. Small pools give erratic effects, showing changes in opposite directions for different portions of the temperature range. It is concluded that the variation of photoelectric effect is intimately connected with the strains produced in the surface by expansion and contraction with temperature.

Characteristics of Photographic Materials in the Ultraviolet

Abstract: In order to develop a method of heterochromatic photographic photometry which can be used to compare line intensities at different wave lengths in the ultraviolet, the sensitivity, speed, contrast, scale, and fogging characteristics of six standard emulsions were studied in the region 4350-2144A. Light from a constant source (mercury vapor lamp or cadmium spark) was dispersed in a high aperture monochromator and the energy distribution was measured by a thermopile and d’Arsonval galvanometer. The light was diminished in intensity uniformly at all wave lengths by means of a diaphragm and screens. Density-log intensity and density-log time curves were plotted at fourteen wave lengths for each emulsion, in some cases for several times of development, and from these the values of intensity contrast and time contrast were measured. All plates showed a decreasing contrast with decreasing wave length, although some showed a minimum at 2800A, with somewhat increased contrast for a short distance at shorter wave lengths. All showed an increased speed at shorter wave lengths over that in the visible as far down as 2500A, when the speed for all except the Schumann plates decreased rapidly. Cramer contrast process plates, with extremely high contrast in the visible, showed greatly decreased contrast at shorter wave lengths, though even here the contrast was greater than that of the other plates. The reciprocity law was found to hold very closely with this plate at all wave lengths above 2500A (and possibly below), over the time and intensity range studied. Eastman Speedway plates showed fairly uniform speed and contrast down to 2500A. These two plates were found excellent for photometry, the first having high contrast and slow speed, and the second low contrast and high speed. They showed the least tendency to acquire chemical fog of any of the plates. The Schumann plates were found unsuitable for photographic photometry, having large variations in sensitivity over their surfaces. Data are also given for several oil-coated emulsions in the region 2700-2144A. Oil can be used not only to increase speed and contrast at wave lengths below 2500, but to make these factors vary less with wave length.