Index of content:
Volume 19, Issue 10, 01 October 1948
19(1948); http://dx.doi.org/10.1063/1.1697891View Description Hide Description
Nickel‐manganese alloys containing 20.1 percent, 21.4 percent, and 25.3 percent Mn in the form of Rowland rings were put into an ordered condition by very slow cooling to 380°C, and long annealing at that temperature. Magnetization curves and hysteresis loops at room temperature showed that the 21.4 percent Mn alloy was very soft magnetically, while the 25.3 percent Mn alloy was relatively hard magnetically. The 20.1 percent Mn alloy was magnetically soft, but not so soft as the 21.4 percent alloy. The magnetic properties of the ordered 21.4 percent Mn alloy changed markedly from room temperature to about 120°C. Induction at 30 oersteds decreases very rapidly, and the maximum permeability goes through a sharp maximum at 91°C. Coercive force and remanence also decrease during the same temperature interval.
19(1948); http://dx.doi.org/10.1063/1.1697892View Description Hide Description
Two methods are described for determining the gain and effective area of an electro‐magnetic horn antenna. The first of these is an experimental method based on the reciprocal properties of antennas. The second method uses data obtained from measured receiving patterns, and from theoretical patterns, for calculation of values for gain and effective area. Comparable values are found from experiment and calculation for eight different horn antennas. It is concluded that the calculations described give satisfactory values of gain and effective area, taking the reciprocity values as standards.
19(1948); http://dx.doi.org/10.1063/1.1697893View Description Hide Description
In this article values for the angles β and ν have been deduced (compare Fig. 3 and Eqs. (2) and (3)). New values have been deduced for the shear angle φ, Eqs. (16), (20), and (21). The increasing of the internal friction and the strain hardening have been considered.
Equations for the work done in cutting per unit of length (6) and (13) have been deduced. For the cutting force Eqs. (14), (15), and (19) have been deduced. A high cutting speed decreases the friction angle τ between the chip and the face of the tool, by which the forces loading the machine tool steeply may be decreased. These forces are very highly dependent on the friction angle τ, since the rake angle α is negative.
A new theory of the formation of a discontinuous chip is presented.
19(1948); http://dx.doi.org/10.1063/1.1697894View Description Hide Description
The nature of the detailed mechanism of the negative Trichel corona pulses in air at atmospheric pressures is reanalyzed on the basis of recent data of English giving the time duration of the pulse and details of its structure. The onset of corona from a negative point is shown to consist of pulses of about 4 μsec. duration with a rise time of about 1 μsec. It is shown to initiate according to the classical Townsend equation, building up a positive space charge of increasing intensity at the point surface, and an electronic and beyond this, an ionic space charge further out in the gap. The intense positive space charge leads to an auto‐accelerative ionizing mechanism with Morton‐Johnson ionization near the point. The magnitude of these space charge fields account for the sputtering previously observed. On the basis of the field distributions which can be deduced, the course of the growth of the visual and electrical phenomena can be traced to the point where the space charge distortion by negative ion formation out in the gap causes the pulse to choke off. New pulses will then start as soon as the field has cleared away the inhibiting negative ion space charge, if triggering electrons are present. The rate of removal of the negative ions depends on the clearing time for negative ions across the gap, and the rate increases as the applied potential increases. The duration of the pulse depends on the rate of electron production by the auto‐accelerative process and the negative ion accumulation appears to require nearly the same total number of negative ions to choke itself off as does the positive corona. The axial space charge gradients account for the constrictions and flaring characteristics of the discharge. These findings are correlated with the coronas in very pure N2 and H2. The observed near equality of the thresholds for positive burst pulse corona onset and Trichel pulses is analyzed and shown to be caused only by a fortuitous and exceptionally high work function of the metal point for electron emission by positive ion bombardment in the presence of O2. The difference in positive and negative corona currents at the same potential above onset in the linear regime is ascribable to differences in ion mobility, since both signs of pulses give about the same number of ions. The character of the positive point corona is briefly analyzed by means of the same criteria of field distortion used for the negative corona and the transient character of the streamer pulses is described through a diagram of successive field distortions. In comparing positive and negative pre‐onset coronas, it is emphasized that despite similarities introduced by the choking off of the discharge through space charge accumulations, the positive point removes electrons quite effectively, giving a field distorted by only one sign of space charge, while the negative point has a much more complicated distortion caused by accumulations of positive and negative ions. The negative point has also a much smaller sensitive volume than the positive point, which introduces differences caused by lack of triggering electrons, making the negative point useless as a Geiger counter.
19(1948); http://dx.doi.org/10.1063/1.1697895View Description Hide Description
In the preparation of alloys by sinteringmetalpowders, the alloy is formed by the diffusion of metals into each other. The distribution of metallic powder in space is expressed by means of a triple series; this series is used to obtain the solution of the diffusion equation. The resulting formula gives the concentration of metal as a function of space, time, temperature, and particle size. Sample calculations made for one‐to‐one atomic proportion nickel‐copper alloys are in general agreement with experimental results.
19(1948); http://dx.doi.org/10.1063/1.1697896View Description Hide Description
A method for measuring absorption and velocity of ultrasonic radiation in the frequency range from 5 to 100 Mc by use of pulse techniques is described. Results of such measurements performed on polycrystallinemagnesium and aluminum as a function of frequency and grain size show that the absorption coefficient varies linearly with frequency and inversely with grain size. The scattering power of a metal, and the fidelity with which an ultrasonic pulse is transmitted is shown to depend on the elastic constants of the single crystals. Criteria are established for evaluating the fidelity of pulse transmission in cubic and hexagonal metals, and figures of merit for many such polycrystalline metals are given.
19(1948); http://dx.doi.org/10.1063/1.1697897View Description Hide Description
Bridge erosion is the transfer of metal from one electrode to the other which occurs when an electric current is broken in a low voltage circuit which is essentially purely resistive. It is associated with the bridge of molten metal formed between the electrodes as they are pulled apart, and more specifically with the ultimate boiling of some of the metal of this bridge before the contact is finally broken. This paper is concerned with fundamental studies of this molten bridge and with empirical measurements of the transfer of metal.
From known physical constants one calculates that, when the melting point is reached at a current I, the diameter of the area of contact of two electrodes is about 1.5×10−5 I cm for silver,copper, or gold, and about 7.5×10−5 I for platinum or palladium. When the maximum temperature of the molten bridge reaches the boiling point the mean bridge diameters are roughly 4×10−5 I and 20×10−5 I respectively. Experimental tests have been made of relations entering into these calculations.
On breaking a contact about 6×10−14 I 3 cm3 of metal is transferred from the positive to the negative electrode. This represents about 100 percent of the volume of the molten bridge for silver,copper, or gold, and about 0.5 percent for platinum or palladium. The amount of transfer can be decreased, and even its direction reversed, by heating the negative electrode, and by other means. Calculations have been made of temperature distributions in the neighborhood of a contact and a theory has been developed to account for the reversal of direction of transfer. The theory of the fundamental mechanism has, however, been reserved for later publication.
19(1948); http://dx.doi.org/10.1063/1.1697898View Description Hide Description
Studies have been made of an alloy 88 percent Mn‐12 percent Cu. This alloy when annealed at 925°C and quenched to room temperature has a tetragonal structure of axial ratio 0.97. It has been suggested that such a structure would show twinning along the 101 and 011 planes. Verification of this suggestion is presented.
19(1948); http://dx.doi.org/10.1063/1.1697899View Description Hide Description
The free streamline theory has been applied to the computation of the cavity drag of symmetrical wedges of arbitrary angle. The drag coefficients and cavity widths and lengths are tabulated for a range of both cavitation parameter and internal angle. The pressure distributions computed for two‐dimensional wedges have been taken to be approximately correct for cones of revolution of the corresponding internal angles. Under this assumption, drag coefficients for right circular cones of arbitrary internal angle have been computed and are tabulated for a range of both cavitation parameter and internal angle.
19(1948); http://dx.doi.org/10.1063/1.1697900View Description Hide Description
When high frequency longitudinal and transverse sound waves are sent through a multicrystalline rod of metal, attenuation losses result because of scattering and diffusion of sound waves by the grains. When the grain size is less than one‐third of the wave‐length, these losses are due to Rayleigh fourth power law scattering and are proportional to the grain volume. The scattering factor depends on the anisotropy of the elastic constants. Two different factors are obtained, one for shear waves and one for longitudinal waves. These factors have been evaluated for cubic and hexagonal metals. From the measured elastic constants the only metals with a low loss are aluminum,magnesium, and tungsten. The calculations indicate that the losses for aluminum and magnesium are about equal for longitudinal waves, but for shear waves magnesium has a very low shear loss. It has been found experimentally that magnesium has nearly as low a loss as fused quartz.
Experiments with higher frequencies show that when the wave‐length is one‐third of the grain size or less, the transmission process becomes a diffusion process similar to the propagation of a heat wave. The grain sizes determine the mean free path, and the loss becomes inversely proportional to the grain diameter. An approximate formula for diffusion losses has been obtained which agrees closely with the experimental values.
19(1948); http://dx.doi.org/10.1063/1.1697901View Description Hide Description
When a body is situated at sufficiently great altitudes (about 200 km or above) it is exposed to impact by the meteorites which enter the earth's atmosphere. A preliminary attempt is made to estimate the probability that a body situated in the vicinity of the earth will be hit by a meteorite and, when hit does occur, to estimate the metal plate thickness necessary to prevent perforation by the impact of meteorites of different sizes. For stainless steel skin thicknesses ranging from 0.05 to 0.02 inch it is necessary to consider meteorites as small as those corresponding to magnitude 8 to 11, respectively. In general, however, it is found that for meteorites which are large enough to present a perforation hazard the probability of a hit is negligibly small, particularly if the body is not exposed to meteoritic impact for excessively long periods of time.
19(1948); http://dx.doi.org/10.1063/1.1697902View Description Hide Description
The equations for general steady motion of a perfect gas are expressed in terms of a reduced number of basic dependent variables. Neither constant entropy nor constant flow energy is assumed throughout the flow, but only along individual streamlines. The basic dependent variables used are the ``reduced velocity'' vectorand the logarithm of the pressure, lnp. The resulting form of the dynamic equation isand that of the continuity equation isrepresenting four equations in four unknowns. The fundamental characteristic and shock relations are also expressed in terms of these reduced variables.
19(1948); http://dx.doi.org/10.1063/1.1697903View Description Hide Description