A. W. Lawson

New Device for Obtaining X‐Ray Diffraction Patterns from Substances Exposed to High Pressure

A device is described for obtaining powder diffraction patterns of thin sheets pressed between opposing diamond pistons. The pressure, initially applied by a small auxiliary laboratory press, is clamped onto the sample by a lock‐nut. The whole unit is designed to replace the sample mount of a standard x‐ray diffraction unit. Sample patterns taken on two of the high pressure modifications of bismuth are exhibited.

An Absolute Noise Thermometer for High Temperatures and High Pressures

A null device for determining the ratio of two absolute temperatures with an accuracy of 0.1 percent is described. This device balances the mean‐square fluctuations in voltage across the terminals of two resistors arising from thermal agitation at the temperatures to be compared. The ratio of the resistances, when the noise voltages from the two resistors are equal, determines the ratio of their absolute temperatures. The limitation on the ultimate accuracy of the ratio of temperatures so determined is imposed by the relative value of the first and second moments of the voltage fluctuations observed, which in turn depends on the length of time utilized in averaging the fluctuations. With the apparatus described below, an observation time of two minutes is adequate to guarantee the accuracy claimed above for high temperatures, provided certain elementary precautions in constructing the noise thermometer are observed. Numerous tests have been conducted in the neighborhood of 1000°K which substantiate these ...

Microwave Filters Using Quarter-Wave Couplings

This paper presents a method of designing bandpass and band-rejection microwave filters by appropriately transforming lumped-element filters. Such microwave filters are realized physically as chains of resonant elements (either cavities or irises), coupled by quarter-wave sections of line. A structure of this type has a number of practical advantages over other types of filter structures, such as a chain of directly coupled cavities, because it permits the construction of the resonant elements as separate units, and yields more liberal tolerances on the dimensions of the coupling irises. A comparison is made of the theoretical and measured characteristics of an experimental four-cavity filter.

Energy Transfer in the Earth's Mantle

Several processes of energy transfer in the earths mantle are examined: (1) gray radiation, (2) ambipolar diffusion of electron-hole pairs, (3) exciton transfer, and others. Using Rikitakes values of the electrical conductivity and correcting for the frequency dependence of the conductivity and the variation of the index of refraction with depth, the values for the radiative contribution to the heat flow are found to be appreciably larger than those estimated by Clark. Estimation of contributions from the electronic processes shows that conduction by excitons may play an important role in the lower parts of the mantle.

An X‐Ray Camera for Obtaining Powder Pictures at High Pressures

Two methods are described of obtaining x‐ray powder diffraction pictures of materials subjected to high hydrostatic pressures. Both techniques have been successfully employed to identify simple high pressure structures at pressures ranging up to 15,000 atmos. The essential feature common to both schemes is to encase the sample in a small beryllium bomb which serves to retain the pressure but permits the x‐rays to be transmitted to the sample and diffracted to an external recording film. Sample data on cerium are given to illustrate the possibilities and limitations of this technique.

High‐Pressure Microwave Window

Two conically tapered single crystals of Al2O3, with small ends abutting, are used to simultaneously effect a high pressure seal and provide a matched transformation from a standard 1‐cm circular wave guide at atmospheric pressure to a circular wave guide terminated by a high‐Q cavity resonator with an internal hydrostatic pressure up to 104 bars.

Note on the Efficiency of Radiation Shields

The familiar problem of radiation shields is analyzed by matrix algebra. A general method is developed for the calculation of the efficiency of thick radiation shields of infinite thermal conductivity. If the shields are thin, the method leads to an explicit, general expression for the efficiency of n radiation shields. It is found that, in this case, the ratio of the power required to maintain a radiating surface at temperature T with n thin shields to the power required to maintain the same temperature without shields is given by [{Σ lim 1n(1emk+1eml)}−(n−1)]−1, where emk and eml are emissivities of the two sides of the mth shield.

Extension of De Barr's Analysis of Radiation Shielding

DeBarr has derived a formula for the efficiency of n thick radiation shields with infinite thermal conductivity and diffuse surfaces surrounding a black body source. This analysis has been extended to the general case when the original source has an arbitrary emissivity. A simple procedure for calculating explicitly the temperature of any shield is given, and simple numerical examples are cited. The efficiency of shields which reflect partially diffusely and partially specularly is also discussed.

Proportioning Temperature Controller

Unbalance voltage from a potentiometer is amplified by a simple circuit employing a 60‐cycle polarized interrupter, and used to control the extent of the on‐off cycle of a furnace. Automatic cold‐junction compensation and a sensitive, temperature‐indicating circuit are incorporated. The circuit responds to a change in input voltage of less than 4 microvolts. The long‐term stability is limited by changes in 60‐cycle pick‐up and aging of components to about 40 microvolts.

A Radiographic Method of Dilatometry

A method of dilatometry using x‐ray illumination has been developed and applied to measurements of compressibility. Metal samples, 10 in. long and 316 in. in diameter, are subjected to pressures up to 10,000 atmospheres. Observations of the length of the compressed sample are made by causing x‐rays to cast a shadow on a photographic plate. The changes in length of the shadow are then determined with a comparator. This technique, which has been used to measure the compressibilities of aluminum, iron, and copper, is easily adaptable to remote measurements of any type involving changes of length, e.g., thermal expansion.