Volume 26, Issue 8, August 1955
Index of content:
26(1955); http://dx.doi.org/10.1063/1.1715307View Description Hide Description
The application of an automatic circuit for the periodical compensation of drift in dc amplifiers is described. The circuit consists principally of a relay and memory condenser and is analogous to that described by Offner [Rev. Sci. Instr. 25, 579 (1954)]. The difference is mainly in the application. Whereas Offner used the circuit to restore the balance in differential amplifiers, here it is applied to the drift compensation of an unbalanced straightforward or feedback dc amplifier.
26(1955); http://dx.doi.org/10.1063/1.1715308View Description Hide Description
The paper describes the use of thin foils of Be, Mg, and Al as filters in the 50 to 500 A spectral region. Mention is made of various procedures for the preparation of plastic backing films. Included are illustrations which show how the metallic filters may be utilized to exclude visible light, to sort out overlapping orders, and to transmit narrow bands of radiation by reducing the intensity of undesirable portions of the spectrum. For each of the metals and for a plastic material (Zapon), plots showing the linear absorption coefficient as a function of wavelength are also given. The absorption curves facilitate the choice of the appropriate filter and its thickness.
26(1955); http://dx.doi.org/10.1063/1.1715309View Description Hide Description
Recent interest in slip‐flow phenomenon as applied to high‐altitude supersonic flight problems has revived experimental activity aimed at determining slip‐flow boundary conditions and evaluating empirical reflection and accommodation coefficients. A unique rotating cylinder apparatus has been designed and constructed for studies involving rarefied gas‐surface interactions over a wide range of flow characteristics extending from continuum conditions to the free molecular flow region. In addition to the above studies the equipment can be used to generate a shear flow field essential to the development of instrumentation necessary for the measurement of rarefied gasflow boundary layers and other nonuniform flow conditions. The flow requirements and design considerations which govern the geometry of the system, the choice of vacuum pumping equipment, vacuum instrumentation, and the controlled speed centrifuge drive mechanism are discussed. The remotely controlled floating torsion balance system is described and the mechanical precision of the entire equipment is indicated.
26(1955); http://dx.doi.org/10.1063/1.1715310View Description Hide Description
A general equation for calculating the neutronenergy spectrum from proton recoil counting data is derived. This equation is optimized subject to a constant total error to determine the most favorable experimental conditions. Feasible accuracy is illustrated by presenting a family of curves of the counting time (required for one point of a spectrum) plotted against total error with source intensity as a parameter. The calculations apply only to a ``good geometry'' experiment with a point source.
26(1955); http://dx.doi.org/10.1063/1.1715311View Description Hide Description
A triple‐coincidence‐type proton recoil spectrometer has been developed for measuring neutron spectra of reactors and artificial neutron sources. The instrument is composed of a triple‐section proportional counter with suitable radiator‐absorber wheels, amplifiers, electronic pulse‐height selectors, and a triple‐coincidence circuit. Considerable effort has been made to obtain maximum sensitivity by optimizing the radiator and absorber thicknesses and the counting times, and by minimizing the background effects. Under these conditions the energy resolution obtained approaches that obtained by photographic plates. The spectrometer was calibrated by measuring monoenergetic neutrons from the O. R. N. L. Van de Graaff generator. Typical examples of spectra measurements are included.
26(1955); http://dx.doi.org/10.1063/1.1715312View Description Hide Description
In some scintillation counter applications it is impracticable to place the photomultiplier tube in close proximity to the scintillation crystal; light guides are therefore employed. These guides produce considerable attenuation, so that photomultiplier signal‐to‐noise ratio becomes low. Factors affecting attenuation in light guides are mentioned in this paper; in particular, the effects of various preparations of the guide surface are considered.
26(1955); http://dx.doi.org/10.1063/1.1715313View Description Hide Description
Using a combination of well‐known techniques, it has been possible to develop a magnetically compensating, ultra fine, high sensitivity, zero deflection electron beam probe method with automatic recording for measuring fields in glow discharges applicable to a wide variety of similar problems. Fields ranging from 1000 down to 0.2 volts per cm have been measured in one‐twentieth or less the times required with previous techniques and with a gain in sensitivity by a factor of ten or better. Wall potentials can simultaneously be recorded. The method is limited to pressures below about 3 mm for 10 cm diameter glows by diffusion of the beam, but correspondingly greater pressures can be used in smaller tubes or with higher energy (say 50 kv) electron beams. The limitations imposed by the effect of the disturbing magnetic field on the discharge are indicated and examples are given of data taken in a glow discharge.
26(1955); http://dx.doi.org/10.1063/1.1715314View Description Hide Description
Portable inexpensive equipment for maintaining the volt in a laboratory to 0.002% with saturated standard cells is described. The equipment consists of a temperature‐controlled air bath for the cells, and a thermofree potentiometer for intercomparing cells with reasonable accuracy.
26(1955); http://dx.doi.org/10.1063/1.1715315View Description Hide Description
Two small thermostats for use in the temperature ranges −40°C to −70°C and −70°C to −150°C are described. The controlling element in both cases is a thermocouple used in conjunction with a potentiometer and galvanometer‐photomultiplier amplifier. The higher‐temperature thermostat uses an acetone bath as the immersion medium while the lower temperature one uses a metal block for this purpose.
Temperature control to within 0.01°C may be attained with both thermostats if reasonable care is taken in correcting for the normal drift in the potentiometer calibration which occurs over periods of time greater than an hour.
26(1955); http://dx.doi.org/10.1063/1.1715316View Description Hide Description
A micromanometer is described which satisfies the requirements for measuring laminar or turbulent boundary layer profiles in incompressible flow. It is a design based on the U‐tube principle and has a demonstrated accuracy of ±0.0002 in. H2O for pressure differentials less than 0.8 in. H2O and ±0.001 in. H2O for measurements in the range 0.8 to 24 in. H2O. The lag time of the instrument is small, resulting in economical use of testing time. Results of experiments to find an optimum manometer fluid and the manometer accuracy are included. Measurements of temperature effects show good correlation with theory. Typical boundary layer profiles measured with the micromanometer exhibit a high degree of consistency and lack of scatter.
- Laboratory and Shop Notes