摘要:

It is now well known that a cylindrical helix, when excited at frequencies corresponding to wavelengths comparable to the length of one turn of the helix, can radiate a sharp beam along the axis over a wide frequency range (about one octave). It is shown in the present communication that if the helix be conical instead of cylindrical (the diameter varying along the length of helix), then the axial mode of radiation can be maintained over a much wider band of frequencies. The radiation pattern of a conical helix, 60 cm diameter at the base, tapering linearly to 20 cm at the top in 10 turns within a height of 112 cm (with the ``ground'' provided by brass disk of 100 cm in diameter) has been studied experimentally. It is found that the axial mode of radiation is maintained from 150 Mc/sec to 450 Mc/sec. By increasing the number of turns, the band width can be considerably increased. Assuming a linear current distribution, theoretical expressions have also been deduced forE&phgr;andE&thgr;for a conical helix. Some modifications of the simple conical helix, such as may have special applications, are indicated.

ISSN:0021-8979    

DOI:10.1063/1.1721328

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

Tension tests to fracture under superposed hydrostatic pressure reaching to nearly 30 000 kg/cm2have been conducted on five polycrystalline metals, Ni, Ta, Cb, Mo, and W, two brittle single metallic crystals, Sb and Cu5Zn8, and two brittle amorphous materials, ``Melmac 404'' and B2O3glass. All these materials exhibit ductility at the highest pressure; the largest increases are for the polycrystalline metals of which the largest is W, which breaks brittely at atmospheric pressure but with 81 percent reduction of area at the highest pressure. The shape of the strain hardening curves varies among the five metals from abrupt rise with long tailing off to long gradual rise with comparatively abrupt falling off, in the order W, Mo, Ta, Cb, and Ni. The character of the fracture varies continuously with pressure and is different for the different metals. Of the brittle single crystals Sb has a reduction of area of 55 percent at the highest pressure. The plastic, Melmac 404, gives the smallest increase of ductility, a reduction of area of only 14 percent at the maximum. This small value is doubtless associated with the absence of internal crystal slip planes. B2O3glass gave a reduction of 87 percent; this is largely a phenomenon of plasticity, but there is an appreciable component of viscosity. Both amorphous materials showed an increase of density after pulling under pressure instead of a decrease.

ISSN:0021-8979    

DOI:10.1063/1.1721329

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

When a specially designed field cathode assembly was inserted in a commercial electron microscope which was appropriately modified for the purpose, the geometry of the needle shaped emitter was obtained from micrographs of several of its profiles at various stages of emitter fabrication and experimental use. An investigation of several methods of fabrication revealed that a smooth, simple, and relatively stable tungsten emitter geometry resulted from a refinement of the methods of Benjamin and Jenkins which combines the electrolytic etch and the smoothing action of surface migration. The electric field at any point on an emitter surface was calculated when the emitter geometry was precisely fitted with one equipotential surface from the family surrounding a charged, isolated sphere‐on‐orthogonal‐cone. A theoretical surface distribution of current density for a typical emitter was derived from the calculated field distribution and the wave‐mechanical field emission theory. From this result was calculated a value of the emitting area which was in good agreement with experiment.

ISSN:0021-8979    

DOI:10.1063/1.1721330

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

The use of silver chloride as a material for ``photoelastic'' stress analysis offers the possibilities of studying both elastic and plastic states of stress in a crystalline material on either a micro‐ or macroscale. It is necessary, however, to relate quantitatively the stress state and the observed relative retardation and extinction angle. In this paper these relationships are developed from a general theory of stress birefringence, according to a stress‐dependent hypothesis. This hypothesis and the resulting analytical relationships are experimentally vindicated by measurements made on a variety of single crystal specimens of silver chloride tested in simple tension in the elastic and plastic stress ranges.

ISSN:0021-8979    

DOI:10.1063/1.1721331

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

The following relates to moving objects immersed in a perfect gas otherwise at rest. The momentum of the resulting gas flow is defined. It is shown that it ultimately approaches a definite magnitude as the object assumes a steady motion, said magnitude being independent of the history of the previous motion.This result is applied to the two‐dimensional flows of airship theory, and thus the airship theory is extended to include compressibility effects to a limited extent.

ISSN:0021-8979    

DOI:10.1063/1.1721332

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

The spread of electron beams of rectangular cross section is studied (a) in a field free space, (b) in a reflection space. An experimental method is tried to trace the electron paths from the excited gas molecules left behind by directly photographing the beam in a high vacuum. The results are checked with published theoretical calculations in case (a), and with results obtained through an iterative method in case (b).

ISSN:0021-8979    

DOI:10.1063/1.1721333

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

Since 1941 we have been working on the development of new types of cathodes, using elements such as barium or thorium as emitters. If provided with a suitable base, these elements are able to migrate and, acting as an atomic layer, reduce the work function of the base material. By introducing a separate supply source below the emitting surface, a work‐function‐reducing substance will be caused to migrate to the cathode surface through small capillaries. Evidently a wide range of new types of cathodes is feasible, since the independently stored emitter of a metal capillary cathode can be held at a temperature different from that of the cathode surface. The subject of this paper is the description of some of these cathodes.

ISSN:0021-8979    

DOI:10.1063/1.1721334

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

When a static magnetic field is applied in the longitudinal direction to a wave guide containing a ferrite material, the medium within the guide becomes anisotropic. It has been shown by Polder that when a microwave field is propagating within such a medium, the relation between the components ofBandHis given by:Bx=&mgr;Hx−j&mgr;′Hy,By=j&mgr;′Hx+&mgr;Hy,Bz=Hz, where &mgr; and &mgr;′ depend on the static magnetic field. Using these equations, an investigation of the wave‐guide modes is made. Formal solutions are obtained for guides having circular symmetry. These solutions depend on the solution of some rather involved eigenvalue equations which determine the propagation constants, and which probably can be solved only by numerical methods.

ISSN:0021-8979    

DOI:10.1063/1.1721335

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

A nonstatistical differential equation is derived for the mean square response of linear systems to random inputs. This equation can be used to determine analytically the steady state and transient response of systems with constant coefficients. It is also applicable to time dependent systems. While the solution cannot in general be obtained analytically in this case, the equation is of the type that can be immediately mechanized on analog computers.

ISSN:0021-8979    

DOI:10.1063/1.1721336

出版商:AIP    

年代:1953

数据来源: AIP

摘要:

A method is described for measuring the temperature variation of the low frequency fluctuations of emission current (flicker effect) in diodes with oxide‐coated cathodes. In order to vary the cathode temperature and at the same time avoid the effects of space charge and the variation due to changes of the mean current, it is necessary to operate the test diodes under retarding field conditions. The results show that the majority of tubes exhibit minimum fluctuations at a cathode temperature in the region of 1000°K, although both rising and falling temperature characteristics are encountered. It is concluded that the flicker effect is in fact a combination of at least two separate phenomena. Finally, the various theories are examined in the light of the new information, and none is found to be completely satisfactory.

ISSN:0021-8979    

DOI:10.1063/1.1721337

出版商:AIP    

年代:1953

数据来源: AIP