DOI:10.1049/pi-2a.1953.0001

出版商:IEE    

年代:1953

数据来源: IET

DOI:10.1049/pi-2a.1953.0002

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A free-space method for the measurement of permittivity at millimetre wavelengths is described. This method utilizes the microwave analogue of the optical spectrometer, electromagnetic horns with lenses taking the place of the collimator and telescope of the optical instrument.At these wavelengths aperture dimensions are necessarily comparable with the radiated wavelength, and diffraction effects are pronounced. The design of the spectrometer and possible aperture illuminations are discussed using the concept of an angular spectrum of plane waves.Permittivities are measured using sheets of material on the spectrometer and studying the reflectivity with angle of incidence for different polarizations. Minima at which destructive interference occurs together with the Brewster angle are determined, and accurate values permittivity deduced.The effects of dielectric loss on the minima are investigated and are found to be small for normal values of loss tangent. The loss in the dielectric is assessed from the shape of the minima and by measurements of transmission coefficients. Diffraction effects on the measurement are discussed, whence it appears that the method should give identical results in both Fresnel and Fraunhofer regions of diffraction.Results for a whole range of specimen sizes and spacings in the instrument substantiate these conclusions, and show that the measurements of permittivity are accurate within ±0.5% in both regions of diffraction. An extension of the method to liquids is also described.

DOI:10.1049/pi-2a.1953.0003

出版商:IEE    

年代:1953

数据来源: IET

摘要:

Aggregated granular semi-conductors can display high permittivities and dispersion effects which vary with temperature and voltage. This behaviour need not be characteristic of the granule material: can be a consequence of its conductivity and of a particular kind inhomogeneity of the aggregate arising from the contact structure the intergranular boundaries. The dielectric properties of certain sintered magnesium ferrites can be accounted for in this way.

DOI:10.1049/pi-2a.1953.0004

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A detailed study is made of the permittivity and power factor of a number of insulating materials over the whole range of conditions encountered in electrical practice, including frequencies from 10 c/s to 24 000 Mc/s and temperatures from −35° C to +240° C in appropriate cases. The materials fall into the following groups:(a) Silicones and other silicon-oxygen structures including mica and glass.(b) Non-polar plastics including polystyrene, polyethylene, polytetrafluorethylene, etc.(c) Pure synthetic resins of the phenol-formaldehyde and anilineformaldehyde classes.(d) The polar plastic Kel F, a chlorinated derivative of the nonpolar polytetrafluorethylene.The main features of the dielectric behaviour of these groups are explained in terms of known features of their structures.

DOI:10.1049/pi-2a.1953.0005

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The temperature coefficient of capacitance of a metallized-mica capacitor varies with frequency as a result of relaxation processes, and is therefore correlated with the power factor of the mica. The temperature coefficient of commercial capacitors ranges at least from −40 to +100 parts in a million per degree centigrade at 1 kc/s; it may be more negative, by about 40 parts in a million per degree centigrade, at 1 Mc/s. There is usually a non-cyclic variation as well, which may be of the order of 0.1%.

DOI:10.1049/pi-2a.1953.0006

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper describes the results of measurements of the power factor and relative permittivity of purified, uncompounded, natural rubber within the temperature range from −50 to +40° C and within the frequency range from 50c/s to 1 Mc/s. The effect of exposure to water vapour on the dielectric properties is contrasted with the effect of a similar exposure on the properties of crude rubber.The methods of purification of the rubber and preparation of the test specimens are described, and the dielectric properties of vulcanized purified rubber are briefly indicated.The Introduction describes the nature of the rubber hydrocarbon and its natural impurities, and mentions the methods of compounding and vulcanizing rubber for electrical purposes.

DOI:10.1049/pi-2a.1953.0007

出版商:IEE    

年代:1953

数据来源: IET

摘要:

The paper gives the results of an investigation of the dielectric properties of two chlorine-methyl substituted benzenes, pentachlortoluene and 1,2,4-trimethyl 3,5,6-trichlorbenzene, and of pentaiodobenzene, over the frequency range 50–4.8×1010c/s, and discusses the observed behaviour in terms of modern dielectric theory. It was thought possible that these materials would exhibit resonance absorption owing to rotational molecular oscillation at the upper end of this frequency range. This phenomenon was not observed, but study of the dipolar absorption in the chlorine substituted compounds showed this to be characterized by a very narrow band of relaxation times, approximating closely to a single relaxation time. A quantitative estimate of the breadth of the relaxation time distribution was made.Directional permittivities of crystalline pentachlortoluene made at low frequency and at optical frequencies showed this material to be strongly anisotropic.

DOI:10.1049/pi-2a.1953.0008

出版商:IEE    

年代:1953

数据来源: IET

摘要:

Acidity and resistivity measurements have been used to judge the condition of used oil and have revealed a few oils with conductivities which are abnormally high when compared with their low acidities. Examination of such oils by filtration, separation and chemical extraction, together with attempts to simulate the high conductivity by additions of pure substances to new oil, suggested that low resistivity was associated with high iron content.To determine the extent of the problem a survey was carried out on oil samples from Grid and generator transformers with ages ranging between one and 24 years. A statistical analysis of the resistivities, acidities and iron contents of these oils showed that the combined presence of iron compounds and acids, which is more often encountered generator transformers, can give rise to high conductivity.Synthetic insulating fluids, and particularly the chlorinated diphenyls, may be used where low fire risk or high operating temperatures are required. Brief mention is made of a laboratory investigation of these substances which gives some idea of the problem involved when hydrochloric acid is released by a spark occurring in them.

DOI:10.1049/pi-2a.1953.0011

出版商:IEE    

年代:1953

数据来源: IET

摘要:

A modified Schering bridge which, whilst simple in operation, will measure the power factors of insulating oils with a high order of accuracy has been designed and constructed. The power factors of insulating oils are often very low, e.g. 0.00005 or less at room temperature, and considerable care must therefore be taken to avoid small errors which may arise from deficiencies in the electrical circuit, the oil test cell and the measuring technique. An oil test cell available commercially has been modified for use in an oil bath, so that accurate temperature stabilization is rapidly attained, and a method of cleaning and filling the cell has been evolved. Details of cell corrections are given. The procedure described in the paper enables the 50-c/s power factor of an oil to be measured with an error of approximately ±3% ±0.000015.

DOI:10.1049/pi-2a.1953.0012

出版商:IEE    

年代:1953

数据来源: IET