Volume 22, Issue 12, December 1951
Index of content:
22(1951); http://dx.doi.org/10.1063/1.1745818View Description Hide Description
A spectrophotometer, equipped with photomultiplier tubes, is described for the wavelength range 2500–7000A. The apparatus is intended for the determination of light absorption in microscopic objects and is especially useful for studies of biological material. The guiding principle of the construction is that the object in the course of the measurement should have a fixed position. Some applications concerning studies of small parts of cellulose and wood fibers are given. The weight of the examined samples has been between 10−10−10−12g.
22(1951); http://dx.doi.org/10.1063/1.1745819View Description Hide Description
The potentiometer embodies the principle of the normal fluxmeter method of measuring hysteresis loss but, while retaining the accuracy, enables the measurement to be made in a fraction of a minute. The potentiometer incorporates a number of tappings which are connected to individual contact studs so that by rotating a contact arm the current supplied to a magnetizing circuit can be varied in a cyclical manner between two limiting values. There are intermediate positions of the arm which correspond to intermediate values of current and these values differ by fixed amounts from one cycle of magnetization to the next. The contact arm also forms part of a stud switch so that a search coil can be disconnected from a fluxmeter over the part in each cycle over which the current changes between the intermediate and one of the limiting values. In this way a complete rotation of the arm is made to take a specimen through twelve cycles of magnetization and since the intermediate current values are evenly distributed over a complete cycle the fluxmeter registers a deflection which can be related to the hysteresis loss.
22(1951); http://dx.doi.org/10.1063/1.1745820View Description Hide Description
An instrument for measurement of the low frequency magnetic properties of thin layers is described. The difference signal from two opposed pick‐up coils, one of which contains the sample, is amplified, integrated, and amplified again to cause the vertical deflection of a cathode‐ray oscillograph to be proportional to the intensity of magnetization of the specimen. A signal is applied to the horizontal deflecting plates of the oscillograph to cause it to represent the applied magnetizing field in the region of the pick‐up coils. A large reduction in low frequency noise in the difference signal preamplifier is accomplished by use of a circuit similar to electrometer tube circuits. Noise introduced through integration of sharp peaked signals is reduced by use of first and third harmonic components in the magnetizing field current. A signal equal to noise is produced by a flux change of 75×10−6maxwell.
22(1951); http://dx.doi.org/10.1063/1.1745821View Description Hide Description
The trajectories of an electron with zero canonical angular momentum have been computed with third‐order accuracy for a 1/(1+x 2) axial field shape. By suitably matching the above solutions with a set of similar solutions obtained by shifting the origin, it is possible to obtain the complete zero canonical angular momentum trajectories corresponding to the double lens problem in which the axial field shape approximates rather closely one that is physically realizable. The numerical computations illustrate several typical trajectories. Design considerations are given to illustrate the utility and limitations of the theory.
22(1951); http://dx.doi.org/10.1063/1.1745822View Description Hide Description
A micro‐electrophoresis apparatus based on interferometry is described. The interferometer consists of a modified Jamin arrangement, using only one plate while the other is replaced by a reflecting cell at right angles to the beam, thereby doubling the sensitivity. A cell filling requires 0.4 ml of solution. No thermostat is provided. Details of the optical and electrical equipment are given. The apparatus may also be used for diffusion experiments.
22(1951); http://dx.doi.org/10.1063/1.1745823View Description Hide Description
Air absorption in the conventional x‐ray fluorescence analysis equipment limits detection to elements heavier than calcium. To avoid absorption of the longer‐wavelength x‐rays from lighter elements, a vacuum system, or one filled with H2 or He, is required. A vacuum apparatus has been developed for the analysis of elements from magnesium to titanium. It contains the x‐ray tube for exciting fluorescence; a specimen holder for six specimens; a collimator to limit the fluorescent radiation to a parallel beam; and a single‐crystal, Geiger counter spectrometer to analyze the radiation. A pressurized, double‐film, nitrocellulose window is used for the Geiger counter because the usual mica windows are opaque to radiation from elements lighter than sulfur. For calcium Kα, the increase in intensity resulting from evacuating the chamber is 100‐fold, with far greater increases for the lighter elements. Alkali halides and gypsum were tested as analyzing crystals. An application of the equipment to the measurement of sulphur in oil indicates that 0.5 percent sulfur can be detected with ±0.03 percent accuracy in 5 to 10 minutes.
22(1951); http://dx.doi.org/10.1063/1.1745824View Description Hide Description
The main purpose of this paper is to describe an apparatus for measuring the intensity of the focused electron beam as it passes through the final viewing chamber of the RCA electron microscope Model EMU. A portion of the electron rays serving to form the final image is collected by a metal probe; these electrons are passed to ground through a high resistor. The voltage developed across this dropping resistor is measured; it represents the intensity of the focused electron beam. Uses as an exposure meter in micrography and for metering the current density incident on the specimen are described.
22(1951); http://dx.doi.org/10.1063/1.1745825View Description Hide Description
Construction and filling procedures are described for neutronproportional counters filled to 101‐cm Hg of enriched BF3+20‐cm Hg of A.
Examples are given of the plateau and bias curves obtained for different values of the gas multiplication. Characteristics taken at different times after filling are practically identical, which indicates that no deterioration occurs.
The effect of the electron affinity of the BF3 is discussed, and it is shown how the counter parameters affect the survival probability of the electrons, hence the quality of the pulse size distribution.
It has been verified experimentally that negative ions produced by electron attachment are present in the counter gas.
22(1951); http://dx.doi.org/10.1063/1.1745826View Description Hide Description
A simple circuit for obtaining nuclear magnetic resonance absorption signals has been developed. Conventional modulation of the magnetic field is replaced by frequency modulation of the oscillator, a method which is technically convenient for the accurate measurement of magnetic fields in the range of 1000 to 18,000 gauss. The principles of operation and construction techniques are described in order that the user may readily duplicate results.
22(1951); http://dx.doi.org/10.1063/1.1745827View Description Hide Description
A gamma‐ray counter for liquid or solid samples is described. It uses an RCA 5819 phototube at room temperature with a thallium activated sodium iodide crystal in the shape of an annular ring. The gamma‐ray counting efficiencies of this counter for 2‐milliliter samples of Fe59, Co60, and I131 are about equal to the beta‐particle counting efficiency of a mica‐window G‐M counter when counting thin samples. When the counter is heavily shielded, the background is about 160 counts per minute. The phototube operating voltage is not critical since over a wide range of operating voltage the counting efficiency and background count change only slightly. A directional gamma‐ray counter is also described.
22(1951); http://dx.doi.org/10.1063/1.1745828View Description Hide Description
Expressions are derived for the heat flux q 0 and temperature gradient dT/dh in a Pyrex cylinder (a) in vacuo and (b) in the atmosphere of cold vapor issuing from a liquid helium container. In the temperature range 20<T<300 we find for case (a) that q 0 α T 1.8 and dT/dh α T −0.8, while in case (b) q 0 T and dT/dh is nearly constant.
Design details and operating procedures are given for a 12‐liter liquid helium storage vessel intended to complement our Collins helium liquefier. It can be filled in one day's operation of the liquefier, and ten days later is still half full.
22(1951); http://dx.doi.org/10.1063/1.1745829View Description Hide Description
A Geiger counter x‐ray spectrometer has been designed and constructed suitable for the quantitative study of crystallinity and orientation in polymers and of relaxation phenomena associated with these properties. The instrument is designed for use with crystal monochromatized or filtered radiation, and has such features as Soller slit collimation, means of studying the materials above room temperature, and a Geiger counter monitor on the x‐ray beam. The spectrometer, pertinent techniques, and typical results are described.
22(1951); http://dx.doi.org/10.1063/1.1745830View Description Hide Description
The instrument consists of four hollow, silver spheres 6 mm in diameter, of identical construction; one sphere is painted black, one white, and the remaining two are highly polished. Each is provided with a thermocouple for measuring its temperature, and an internal heating coil for adjusting its temperature. Total environmental thermal radiation intensity is obtained by measuring the heat input to the two spheres of lower temperature to bring the black, white, and one polished sphere to the same temperature when the instrument is exposed in a given environment. The second polished sphere provides for wind velocity measurements after the manner of a sphere anemometer. Measurement of direct solar radiation exclusive of reflected and scattered solar radiation is made by comparing the total radiation in sunlight with that obtained when the spheres are shaded from the sun. The radiant temperature of the environment is obtained by correcting the measured value of total radiation for the effect of solar radiation. A precision of ±4 percent can be realized for the measurement of solar radiation and of ±1.8°C for the measurement of the radiant temperature of the surroundings.
22(1951); http://dx.doi.org/10.1063/1.1745832View Description Hide Description
22(1951); http://dx.doi.org/10.1063/1.1745834View Description Hide Description
An account is given of a simple, reliable, and generally applicable electrical means for accurately measuring and controlling the level of low boiling‐point liquids. The sensitive elements exhibit at least a fivefold voltage difference for nitrogen and at least a five hundred‐fold difference for helium depending on whether they are in the liquid or the vapor. Heat transfercharacteristics for liquid nitrogen are also noted.
22(1951); http://dx.doi.org/10.1063/1.1745835View Description Hide Description
Previous work with large solar furnaces is reviewed. New developments in the design of the 120 in. (3.05 m) solar furnace are discussed, in particular the mounting of the reflector, the parabolizing of the aluminum shell, and the production of a surface of high reflectivity. Close control of the temperature to which a sample is heated is attained by means of an aluminum cylinder which travels along the axis of the mirror and which controls the amount of radiation incident on the sample. Rotating sectors are used for separating the temperature radiation emitted by the sample and the incident radiation from the sun. Heating tests for obtaining molten materials, glasses, and products of crystallization are discussed. Methods are presented for work approximating blackbody conditions at very high temperatures. A method for the accurate determination of melting points of highly refractory samples under pure conditions is presented.
22(1951); http://dx.doi.org/10.1063/1.1745836View Description Hide Description
A spherical electrostaticanalyzer was constructed for use with the Wisconsin cylindrical analyzer in accurate measurements of nuclear reaction energies. The focusing properties of such an instrument have been described by Purcell. It has the unique feature of providing a large solid angle and high resolution for a very small angular spread of the reaction products.
Only a portion of the entire sphere is used. This is 90° along the path of the particles with a 60° azimuthal opening. The mean radius of the gap is 18 inches and the gap spacing is inch. One million volt protons may be focused with a gap voltage of 20.2 kv. The analyzer is designed to fit over the cylindrical analyzer and to accept particles at an angle of 135° from the bombarding beam.
Alignment, focus, and resolution tests show that the difficult machining of the plates has been done satisfactorily and that the analyzer is properly aligned. A shutter allows small sectors of the total gap to be tested individually. A sharp focus is obtained and all sectors focus particles of the same energy for a given gap voltage.
Resolution tests and calibration are made using monoenergetic protons scattered from thin targets of heavy elements. A plot of the number of particles entering the detectorvs the voltage across the spherical gap gives the resolution. With 1‐mm object and image apertures the measured energy resolution is 650. The measured effective solid angle with this source size and resolution is 3.2×10−3 steradian.
22(1951); http://dx.doi.org/10.1063/1.1745837View Description Hide Description
A new instrument, the cloud‐ion chamber, combining the functions of an ionization chamber (utilizing free electron collection) and the Wilson cloud chamber in the same gas volume has been operated successfully for a sensitive time in excess of 1000 hours. From studies of the gas mixtures with argon as the major component, it is shown that an isoamyl alcohol and argon mixture is a satisfactory chamber filling under the operating conditions used.
The operating characteristics of the cloud‐ion chamber are described.