Volume 16, Issue 10, 01 October 1945
Index of content:
16(1945); http://dx.doi.org/10.1063/1.1707505View Description Hide Description
A review is given of typical deviations from ordinary valence rules by alloys and inorganic compounds. The effect on the electrical properties of a compound of a deviation from stoichiometric proportions is discussed. The value of the information obtained from electrical measurements is illustrated by a discussion of the properties of several semi‐conducting compounds.
16(1945); http://dx.doi.org/10.1063/1.1707506View Description Hide Description
The electrical properties of semi‐conductors are briefly reviewed. The information concerning the structure of these materials which is obtained by electrical measurements is discussed. The most important of the physical‐chemical methods available for the investigation of deviations from stoichiometric proportions in semi‐conducting compounds are illustrated by a discussion of several typical examples.
16(1945); http://dx.doi.org/10.1063/1.1707507View Description Hide Description
This expository article gives a straight‐forward derivation of the classical representation formulas of the three‐dimensional rotation group in explicit (non‐symbolical) form starting with the most elementary representation of a rotation by a linear homogeneous transformation with a proper orthogonal matrix. Apart from Euler's formula (3.4) (which is taken as starting point in Benedikt's previous paper on the same subject) other representations often used in mechanics and physics are derived and the various connections existing between these formulas are established hereby going beyond Benedikt's paper by taking into account the simplest spin representations of the rotation group. The spin representation by the special unitary group is derived in two ways; the second method (Section 6) is based on a general fact (in italics) which seems not to be generally known. In the footnotes some references to the literature are given.
16(1945); http://dx.doi.org/10.1063/1.1707508View Description Hide Description
The plan and construction of a device capable of accelerating electrons to energies as high as 100 million electron volts are described together with the special building to house it. A more detailed description of the device and its construction will appear in the General Electric Review. The accelerator has a pole face 76 inches in diameter, weighs 130 tons, and operates on 60‐cycle current requiring at full load 200 kilowatts. The machine is air cooled and is capable of continuous operation at full voltage. The toroidal glass vacuum tube of oval section has an external diameter of 74 inches and consists of 16 sectors cemented together. At 100 million volts the x‐ray output, measured in a thick‐walled (¼ inch lead) ionization chamber, is 2600 Roentgens per minute at one meter target‐thimble distance and the half‐value width of the x‐ray beam is 2.0 degrees. The penetrating power of the x‐rays in iron at various voltages has been measured and the radiographic possibilities studied.
16(1945); http://dx.doi.org/10.1063/1.1707509View Description Hide Description
The nature of the thermal expansion anomaly in high polymers, known as the second‐order transition, is examined in some detail. It is suggested that below the transition temperature, Tm, polymer chains can expand sidewards but not parallel to their length. At Tm lengthwise expansion becomes prominent, thus accounting for the sudden increase in thermal expansion. Experimental results are presented showing the anisotropicexpansion of oriented polymers below Tm. The transition effect is then treated as a problem in viscous flow, which gives rise to various semi‐empirical plots connecting Tm with applied forces, plasticizer content, and time effects. The brittle point, Tb , involves highly elastic deformation, and is shown to be a fundamentally different test, although Tm and Tb are sometimes numerically equal. Various factors influencing the brittle point are reviewed briefly.
16(1945); http://dx.doi.org/10.1063/1.1707510View Description Hide Description
A new and relatively simple method of measuring the resistance of wires at frequencies between about 200 kilocycles and about 40 megacycles is described. By this method, the test specimen is strung down the center of a metal pipe about ten to a hundred feet in length, depending upon the test frequency, to form a transmission line. One end of the line is short circuited and the other connected to a standard commercial Q‐meter. Readings are taken from the Q‐meter and applied to curves to obtain the desired resistance. This method of measurement appears to be more reliable than the use of coils, and the results agree closely with calculations by skin‐effect formulas. For round wires known to consist of homogeneous material, calculated values of radiofrequency resistance appear to be just as reliable and accurate as experimental values, and are much more easily obtained.
16(1945); http://dx.doi.org/10.1063/1.1707511View Description Hide Description
In the precision determination of lattice constants from back reflection Debye‐Scherrer patterns, one of the systematic errors arises from the effect of absorption in the sample. The magnitude and form of the error is calculated for a cylindrical sample, for both parallel and diverging primary beams, and for a representative variety of values of the absorption coefficient. Calling 2φ the back scattering angle (φ=90−θ), it is found that the error in φ caused by absorption can be set proportional to φ. The form of the absorption correction allows it to be lumped in with the other systematic errors for a linear extrapolation according to either Δd/d =Kφ tan φ or Δd/d=K sin2 φ. There is some preference for the former.
16(1945); http://dx.doi.org/10.1063/1.1707512View Description Hide Description
Some preliminary results are reported on measurements of molecular weight distributions for polychlorostyrene. The following technique of isolating single polymer molecules is used: A precipitating agent, such as propanol, is added to a very dilute (1 ppm) solution of the polymer in benzene, presumably causing the molecules to coil up. Drops of this mixture are evaporated on a silica screen and examined in the microscope. Circular particles ranging in diameter from 15A to 500A and believed to represent single polymer molecules have been obtained on various polymers. The relationship between measured particle diameter and molecular weight is reviewed in some detail. The data appear to favor the concept of a random coil to the extent that particle diameter varies as the square root of the molecular weight. The molecular weight distribution curves obtained are of the expected shape and extent, although the average molecular weights computed from the distribution curves are 4–5 times greater than values measured by an independent method. Finally, the role of electrical charges in stabilizing isolated polymer molecules is discussed.
16(1945); http://dx.doi.org/10.1063/1.1707513View Description Hide Description
The theory of the frequency conversion diagram has previously been presented in this journal. The research work is now extended to include triode tubes, and it is shown in the following that the conversion diagram provides a suitable method of treatment of non‐linear mixers with more than two electrodes. Actually, the greater the number of variables, and the greater the difficulties in a mathematical analysis, the more important becomes this method of obtaining frequency conversion data. A brief description of the measurement equipment is given in the text and typical diagrams and derived curves are shown.