Volume 15, Issue 2, 01 February 1944
Index of content:
15(1944); http://dx.doi.org/10.1063/1.1707402View Description Hide Description
Electromagnetic Velometry. I. A Method for the Determination of Fluid Velocity Distribution in Space and Time15(1944); http://dx.doi.org/10.1063/1.1707404View Description Hide Description
A method is described which permits the determination of the local velocity of flow, as well as of turbulent fluctuations at a given point, or a prescribed series of points in a moving liquid. The method is based on the induction of a potential gradient in the flowing medium as it traverses a magnetic field perpendicularly to its direction. The absence of lag in the process of induction makes the method particularly suitable for the study of rapidly varying velocities and the sharp localization of measurements allows the determination of spatial velocity distributions. In an adaptation of the method for small scale laboratory experiments, the magnetic field is created by a large a.c. magnet in whose gap the conduit is placed, while two minute exploring electrodes pick up the induced e.m.f. at the desired locations. For large scale experiments and field work, a self‐contained unit has been designed which comprises a minute electromagnet with attached pick‐up electrodes. The applicability of this method is demonstrated by velocity distribution curves taken under different conditions and by records of liquid turbulence and transient velocity changes in different forms of flow. This paper is confined to the presentation of the principle involved, to the description of the apparatus, to the discussion of the experimental technique, and to demonstrations of the applicability of the method. A theoretical analysis of the characteristics and limitations of this method will be dealt with in a separate publication.
15(1944); http://dx.doi.org/10.1063/1.1707405View Description Hide Description
A crystallographic analysis of a characteristic shape of particle found in zinc oxide produced by burning zinc vapor has been made with the aid of the electron microscope. This particle consists of four needle‐shaped crystals united at a common juncture. The spatial angles among the four crystals were determined from stereoscopic micrographs with the use of the stereographic projection. The crystals were found to be united by twinning on planes of the form (112).
The Impedance of Short, Long, and Capacitively Loaded Antennas with a Critical Discussion of the Antenna Problem15(1944); http://dx.doi.org/10.1063/1.1707406View Description Hide Description
In Part I curves computed from Hallén's formula for the self‐impedance of a center‐driven antenna of half‐length h and radius a are given for Ω=2 ln (2h/a)=7, 10, 20, 30 for values of h from zero to two wave‐lengths. A simple formula for the impedance of short antennas is obtained; the reactance is shown to agree with that computed from the static capacitance between the two halves of the antenna. The effect of a capacitive reactance connected across the input terminals of the antenna is investigated. It is shown that the resistance at anti‐resonance is reduced, the capacitive lobes of the curve for reactance relatively increased, the inductive lobes relatively very much decreased, and curves for both resistance and reactance shifted to shorter lengths. In Part II the differences between various methods used by different investigators to compute the impedance of an antenna are examined critically. It is concluded that Hallén's formula probably is a better approximation for an antenna consisting of two sufficiently thin ellipsoids each of semi‐minor axis a placed end to end than for cylinders, but that cylindrical antennas driven in various ways can be approximated by using an effective value of h/a and a suitably chosen capacitance across the input terminals.
15(1944); http://dx.doi.org/10.1063/1.1707407View Description Hide Description
Recently calculated values of the input impedance of cylindrical antennas are expressed in a form adapted to transmission‐line equations. Curves are plotted to show the effect of various antenna and line parameters on a circuit consisting of a line with an antenna as load, and application is made to the transfer of power from the line to the antenna.
15(1944); http://dx.doi.org/10.1063/1.1707408View Description Hide Description