One-half Period Research Articles

Heat capacity and thermodynamic properties of p-terphenyl: Study of order–disorder transition by automated high-resolution adiabatic calorimetry

The heat capacity of a sample of zone-refined, high-purity p-terphenyl has been determined from 4 to 370 K in a fully automated high-resolution vacuum adiabatic calorimeter and from 320 to 580 K in a differential scanning calorimeter. The melting point of p-terphenyl is 487.0 K and the enthalpy of fusion is 35.3 kJ/mol. A λ-type solid–solid transition occurs with a peak temperature of 193.55 K. The transition is highly reproducible without observable hysteresis, even after various thermal treatments. In the transition region which spans from 140 to 240 K, the sample reaches a state of thermal equilibrium within a period of one-half to one hour, as normally required in adiabatic calorimetry. These characteristics are desirable for the application of the equilibrium λ transitions as a calibration standard for use in dynamic calorimetry. The behavior of the λ transition at equilibrium is mapped in high temperature resolution, with small temperature increments of measurement down to 0.01 K by adiabatic calorimetry. In the temperature region between the transition and the fusion, the heat capacity of the high-temperature form of the crystal is proportional to the temperature Cp =0.94 T J/K/mol to within 1%. The heat capacity of p-terphenyl in the liquid state above its melting point of 487.0 K appears to be an extension of the heat capacity of the liquid, and the supercooled liquid, state of o-terphenyl above its glass transition temperature of about 243 K.

A theory of thin metal films: electron density, potentials and work function

Electron densities, potentials, and work functions of thin metal films are calculated self-consistently. The planar uniform-background model and the density-functional formalism are used similarly as in the theory of metal surfaces by Lang and Kohn. Electron densities and potentials are discussed for r s = 4 both as function of the position in the film and of film thickness. Numerical results for the dependence of the work function on film thickness are given for r s = 2, 3, 4, 5, 6. As functions of film thickness electron densities, potentials and work functions show oscillations with a period of one-half the Fermi wavelength. The amplitude of the oscillations in potentials and work functions is about 1 eV for one monolayer and 0.1 eV for films of 20 Å thickness. A comparison with non-self-consistent calculations reveals the necessity of self-consistent computations. The relevance of the results to work function measurements and to investigations of thermodynamic and transport properties are discussed. The influence of the film geometry on calculations designed for surfaces per se is examined.

Image subtraction by polarization-shifted periodic carrier

A coherent image subtractor is described that makes use of a periodic carrier that is effectively shifted by one-half period by simple sheet polarizing material. As the system has two input stations, the two transparencies to be subtracted are simultaneously visible to facilitate alignment.

Recording arc discharges at the cathode in models of MHD devices

The analysis of current-voltage characteristics for channel models of MHD devices gives reason to believe that, in specific cases, the uniformly distributed discharge at the cathode is "stripped" and that an eIectric arc appears [1]. It has also been surmised [2] that a "unidirectional" are exists, which penetrates the boundary layer and extends on one side into the cathode glow and on the other side into the uniform current in the body of the plasma stream. These phenomena have been observed both on "cold" [2], and hot electrodes made of silicized graphite [1]. In order to verify these assumptions, a series of experiments was performed on a channel model to detect arc discharges. The work was carried out in a stream of air (T G ~ 3000 ~ K) with an applied electric field (UEF F = 80 V, f = 50 Hz); the temperature range of the operating surface of the electrode T w was from 1200 to 2000 ~ C, and the additive content varied from 0 to 1.2% by weight of the flow rate. The channel mode1 and the measurement circuit are described in [1]. Together withthe usual oscillograms for the current-voltage characteristics (electrode-electrode and electrode-probe types), the experiments also gave the corresponding values of current and voltage displayed on a time scale. Fig. 1 gives a typical current-voltage electrode-probe characteristic for an electrode made of silicized graphite; Fig. 2 gives an oscillogram of the current and voltage displayed on a time scale. The section OD of the electrode-probe oscillogram (Fig. 1) (the potential drop in a gas layer of thickness ~5 mm as a function of the current flowing in the electrode circuit) corresponds to an operating regime in which the electrode under investigation acts as an anode, and the segment OABC corresponds to an operating regime where this electrode acts as a cathode. We see from Figs. 1 and 2, that the arcs are struck at a specific breakdown voltage U~', corresponding to the point B, after which the voltage decreases and the current increases. While the arc is burning, the voltage remains practically constant during one-half period or decreases a little (point C). After the voltage reaches a value corresponding to the point A, the conditions necessary for maintaining an arc regime disappear and the electrode again begins to operate in a distributed discharge regime. Here, as in the initial sector OB, a sinusoidal variation of current and voltage in the electrode circuit is observed. The oscillograms obtained for the variation of voltage and current in the electrode circuit of a channel model for a MHD device correspond in shape to the voltage and current oscillograms of the stabilized arc between the electrodes of a variable current plasmatron. 9 Fig. I

MINIMA AND LIGHT ELEMENTS OF V 453 CYGNI

ABSTRACT Old unpublished times of minima light of V 453 Cygni and new photoelectric times of minima light are presented. The observations show that the light elements published in A Finding List for Obsercers of Eclipsing Variables appear to be slightly in error and that secondary minimum is displaced from one-half period. The new light elements are Min. I = JD2439340.0988 + 3d8898128 E with Min. II occurring at phase . The measured value of e cos is +0.012. It is most probable that 270 < 360 and e < 0.02. These photometric elements are discordant with J. A. Pearce's spectroscopic elements, especially P, T e, and w. The spectroscopy of V 453 Cyg should therefore be re-evaluated. Key words: eclipsing binary - spectroscopic binary - light elements - light minima

Analog study of dead-beat posicast control

A method is presented which eliminates the oscillations and overshoot in a lightly damped servomechanism in a time of considerably less than one cycle of the uncompensated oscillation. The frequency response of a resonant system compensated in this manner can be made flat up through and beyond the resonant frequency of the system. For dead-beat response in one-half period, the frequency response is down only 3 db at the resonant frequency. Systems designed to have dead-beat responses in much less than one-half period have correspondingly wider bandwidths. The posicast method involves splitting up the input excitation into several fragments. Each fragment is applied at such a time that the sum of all the transient terms is equal to zero after the last excitation. The method is general and can be applied to electrical, mechanical, and even pneumatic systems.

A LINEAR RADIO-FREQUENCY MASS SPECTROMETER

A radio-frequency mass spectrometer is described using a system of electrodes similar to a linear accelerator to obtain velocity selection of tin ions. A beam of ions is directed through a series of axial radio-frequency fields with alternate polarity, and the ion with a velocity (proportional to [Formula: see text]) such that it traverses the individual stages in approximately one-half period of the radio-frequency field, acquires maximum energy from this field. A retarding field region prevents all ions, except the ion with maximum energy, from reaching the collector. An ion beam of large cross section is used. With 20 radio-frequency stages a mass resolution of 1% is obtained.

A photoelectric study of the eclipsing binary RT Andromedae.

The dwarf eclipsing system, RT Andromedae, has been studied photoelectrically for two reasons: (I) the photographic technique was found to be inadequate for resolving the light curve of a system of such short period, 0.6289 days, and (2) it seemed suitable for testing a technique of continuous recording of the light received. Three primary and one secondary minima were recorded without reference to a comparison star on an Esterline-Angus recorder attached to an amplifier and a I P2 I multiplier phototube on the ~6-inch refractor of the Lick Observatory. Observations were also made with a galvanometer by the standard photoelectric technique involving the use of a nearby star as a comparison. Analysis of the observations shows RT Andromedae to be a totally eclipsing system with a ratio of radii equal to more than 0.7. Primary minimum occurred at JD 2432443.7820 and the secondary minimum occurred one-half period later within the limits of accuracy of observation. The secondary minimum was considerably wider than the primary minimum; this difference in width led to a value of orbital eccentricity compatible with that found by S. Gaposchkin spectrographically, but ~ appeared to have changed by about 1700 in 1700 revolutions. These tests of the continuous-recording technique showed that the random errors of observation were lower than those produced by standard methods, but the systematic errors were somewhat higher. We may therefore conclude that the continuous-recording technique appears very valuable if a method of determining variations in atmospheric extinction and amplifier sensitivity is incorporated. A definitive solution of the elements on the basis of what would normally have been sufficient observing was precluded by evidence of intrinsic variability in the light of RT Andromedae. Further observations of this interesting system are planned. Lick Observatory, Mt. Hamilton, Cal~.

Soil Aeration as a Factor in Growth and Root Development of Plants.

The soil in which certain aquatic and terrestial angiosperms were growing was slowly aerated daily for a period of one-half to 5 hours by means of compressed air supplied through perforated coils imbedded 8 inches below the surface. Helianthus annuus, Triticum vulgare, Glycine soja, Linum usitatissimum and cuttings of Vitis vulpina were grown in sand and in loam. At the age of 2 months conspicuous differences were apparent in aerated and unaerated plants on the same soil as well as between plants similarly treated but in different soils. As might be anticipated aeration did not produce the same effects in plants grown in loam and in sandy soil. Structural differences occurred in both tops and root systems, but especially the latter. In general, roots in the aerated soils were distinctly fibrous in character, more numerous and longer, forming branches of secondary and tertiary rank. Total surface in aerated roots was twice or more that of control roots, but the root hair zone was smaller. Fewer hairs developed and these in turn were not as long-lived in aerated plants as in the controls due to the more rapid elongation of the aerated roots. Vascular elements a few inches above the root hair zone were less developed in aerated roots. The striking effect of aeration on tops was acceleration of growth in early stages, attributable to increased length of basal internodes rather than to an increase in the number. Contrast in internodal distance diminished noticeably, however, above the median nodes and there was little or no difference in size of internodes near the tips. Elongation continued in controls after aerated plants had started to flower. This response tended to reduce size differences when mature plants of both groups were compared.