Volume 17, Issue 4, 01 April 1946
Index of content:
17(1946); http://dx.doi.org/10.1063/1.1707708View Description Hide Description
A new electrolytic selenium photo‐cell is described and its characteristics given as a function of operating voltage, temperature, external resistance, and time. Data are included indicative of its response to unmodulated, modulated, and monochromatic light; comparison is made with cells of similar type or possessing similar characteristics; and a method of compensating for the dark current of photo‐cells is shown.
An electronic circuit is also described which can greatly improve the voltage output of the photo‐element. It is generally applicable to photo‐conductive and photo‐voltaic cells.
17(1946); http://dx.doi.org/10.1063/1.1707709View Description Hide Description
The experimental methods are described by which rectangular blocks are subjected to two‐dimensional compressional stresses and at the same time flow is maintained two‐dimensional with deviations of less than one percent. Within this range of flow a rather wide range of stress conditions is possible, so that the equations of plastic flow can be examined over a correspondingly wide range of conditions. It is found that two‐dimensional flow is much more sensitive than three‐dimensional flow to shearing instabilities initiated at the edges where there are singularities in the mathematical solution. By proper lubrication it is, however, possible to produce approximately homogeneous deformations in two‐dimensional compression at least up to shortenings to two‐thirds the initial length. It is found that the transverse stress Xx required to maintain two‐dimensional flow is consistently higher than the theoretical value ½Zz ; for some materials the excess above the theoretical value may rise to as much as 20 percent. This excess tends to be less in the early stages of flow, and does not tend to increase beyond strains of 10 percent. The failure of Xx to be equal to ½Zz indicates a failure of the fundamental condition of isotropy of flow; this failure may be demonstrated in other ways. The time rate of primary flow is studied at various points on the strain‐hardening curve. The rate of flow rises rapidly as the stress increases above the limiting strain‐hardening curve. The rate of increase of the rate of flow for a given displacement from the limiting curve is much greater in the early stages of flow. It is this which makes possible the calculation of the rate of propagation of a plastic disturbance from the parameters of the static strain‐hardening curve. Beyond the early stages of flow new time effects begin to appear. The flow loses its smoothness and becomes more and more inclined to be jerky. At high strains, flow may not start at once when the load is increased, but there may be an initiation period during which flow is built up. The application of the results to the generalized strain‐hardening curve is discussed. In view of the failure of isotropy, strictly speaking, a generalized strain‐hardening curve does not exist. Within the strain limits of this paper it is found that both the maximum shearing stress criterion and the ``significant'' stress‐strain criterion apply with errors of the order of 10 percent, the maximum shearing stress criterion being on the whole perhaps somewhat better.
17(1946); http://dx.doi.org/10.1063/1.1707710View Description Hide Description
A method applicable to determining droplet sizes in the interior of an already existing atmosphere of fog or mist is developed and described. The method makes use of the fact that if droplets strike a suitably coated surface without wetting the surface, a track of the contact area is formed. A criterion is derived and experimentally evaluated for relating the droplet diameter and the track diameter. An apparatus for utilizing this method is briefly described.
The Accuracy of Lumping in an Electric Circuit Representing Heat Flow in Cylindrical and Spherical Bodies17(1946); http://dx.doi.org/10.1063/1.1707711View Description Hide Description
In using lumped resistance‐capacitance circuits for studying heat conduction problems the influence of number and size of lumps is important. Several methods of lumping are conceivable for representation of cylindrical or spherical bodies and results of comparative tests show that equal geometrical size of lumps is most accurate. In the various lumps resistance and capacitance have to be in certain definite relationships, which are established in the paper. The influence of number of lumps is also investigated.
Determination of the Temperature Rise and the Maximum Safe Current through Multiconductor Electric Cables17(1946); http://dx.doi.org/10.1063/1.1707712View Description Hide Description
A general method of calculating the temperature rise in multiconductor rubber insulated electric cables based upon the thermal constants of the cable has been derived. The predictions of the theory have been verified experimentally for a number of cables and values of the thermal resistivity constants have been obtained as a function of size and composition of the cable. The results are used to predict the maximum safe current for different size cables to limit the temperature rise at the center to 25°C above an ambient temperature of 50°C.
17(1946); http://dx.doi.org/10.1063/1.1707713View Description Hide Description
The mathematical principles of the classical iterative method of solving linear simultaneous equations are discussed. Basic electrical circuits for setting up an analog of the mathematical relations are given, and a commercial model of a 12‐equation computer is briefly described. The results of solving a number of problems on the computer are given to illustrate its accuracy and speed of operation. It is found that solving sets of 12 equations requires only 1/4 to 1/7 the time required by conventional methods.
17(1946); http://dx.doi.org/10.1063/1.1707714View Description Hide Description
17(1946); http://dx.doi.org/10.1063/1.1707715View Description Hide Description
The corrections for Lorentz and polarization factors for trial and error crystal structure computations are discussed, following the general method recently presented for correcting x‐ray diffraction data used in Fourier syntheses. The Lorentz correction has the form (1/S) csc y̢, where S is a scale constant characteristic of the reciprocal lattice level and whose form varies with the method of recording the data. The scale constant is eliminated by appropriate timing of the exposure. When many trial and error computations have to be made, there are advantages in using a scheme involving a correction factor (csc y̢)½ instead of csc y̢. Tables are provided for these functions, for the polarization correction and for the combined Lorentz‐polarization correction which is used in the special case of zero‐level photographs. Tables are also provided for the functions (h 2+k 2)½ and (h 2+k 2+hk)½, which are useful for computing the argument for the zero‐level corrections of tetragonal and hexagonal crystals.
17(1946); http://dx.doi.org/10.1063/1.1707716View Description Hide Description
Preparation of Electron Microscope Specimens for Determination of Particle Size Distribution in Aqueous Suspensions17(1946); http://dx.doi.org/10.1063/1.1707717View Description Hide Description
Small droplets of the suspension to be examined are deposited from a mist upon a specimen film whose surface has been treated to promote wetting by the droplets. This procedure makes the entire residue left by a whole droplet available for examination and promotes uniform dispersion of the deposit.