โ Scribed by R.H.O.
No coin nor oath required. For personal study only.
Vol. 52, No. 2.)--]n the Betatron, industry has, for the first time, an instrument that will enable it to examine castings and forgings for hidden flaws which have defied detection in the past and caused foundries to make certain of their products oversize or overweight so that the weakest sections would stand up in service. The Betatron is a 20,000,000-volt X-ray machine--a voltage that is accepted as the present-day limit and adequate for most industrial needs. It was invented by D. W. Kerst, University of Illinois physicist, and is being constructed by Allis-Chalmers Manufacturing Company.
The first unit has been installed at the U. S. Army Arsenal at Picatinny, N. J., and will be used to X-ray shells and rocket warheads more than a foot thick. Its rays are capable of penetrating 20 inches of steel in twenty minutes and of revealing ahnost microscopic flaws. Because the X-ray source is small in size, ic is possible to make direct enlargement of an object by placing the film remote from it. This increases the over-all sensitivity of the radiograph and permits magnification of microparts to three or more dimensions. Further, because there is less scattering and, consequently, less fog than with lower-voltage X-rays, the definition and quality of the radiograph are higher.
The Betatron, in lay language, is a 2-winding transformer with a stream of electrons serving as the secondary highvoltage winding. The latter, instead of being made up of the hundreds of thousands of turns of very small wire that would ordinarily be required to produce 20,000,000 volts, consists of a doughnut-shaped, porcelain vacuum tube containing a sturdy filament and a tiny platinum target. This tube, together with a specially designed circular steel pole piece, is inserted into a small air gap in the center leg of the machim"s sleet magnet or core.
When myriads of electrons are shot from the filament in the evacuated tube, the magnetic flux acts on them nmch as though they were moving inside the conventional turns of wire. As the magnet is energized, the flux surrounding the the pole piece builds up and causes the stream of electrons to race round and round several hundred thousand times and at a rate finally approaching the speed of light. All this takes place in 1/720th of a second! At the peak of their travel, current from an electronic circuit upsets the magnetic field and causes the electrons to spiral off and collide with the tip of the small target. The energy given off by the electrons in their collision with the target generatt,s a supply of high-frequency X-rays which penetrate and, emerge front the walls of the vacuum tube as though it were non-existent.
Thus the Betatron, by combining a transformer winding and an X-ray tube, becomes a highly efficient tool for the scientific worker and industry. With it, to mention but one of its applications, it will be feasible to examine heaw' castings and forgings on a laboratory or production-line scale and to reject even those that have the most minor imperfections, thus eliminating the present practice of oversizing. Through study of the defective parts it may be possible to redesign them with a view to reducing their weight to a point where they will still serve their purpose with an ample margin of safety.
R. H. O.
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