X-RAYS were born on the eve of the Spanish-American war, came of age medically in the hospitals back of World War battlefronts, and are finding their place industrially in the din of today’s drive to forge an impregnable America. In the last few months a half dozen of the most potent X-ray machines yet devised, their million-volt electronic glance capable of peering through eight inches of steel, have been put to work in factories and foundries. While these giants of the breed are detecting deep-hidden flaws in battleship armor or high-pressure boiler drums or testing welds eight inches thick, smaller members of the family are turning their 5,000-volt seeing eyes upon thin aluminum sheet for fighter planes, producing micro-radiographs of metal structure and measuring stresses in magnesium, brass and aluminum by X-ray diffraction technique. The range of usefulness for X-rays is as wide as American industry itself. In a western packing house an X-ray inspection unit tells good oranges from bad as they pass beneath a fluoroscopic screen. In others, X-rays detect foreign bodies or spoiled foods in containers. All along the line from the low voltage instruments up to the million-volt “gun,” X-rays have their particular jobs to do. Most spectacular, of course, is the biggest. Big in potentialities, though not in dimensions; for the million-volt machine has been tailored by the geniuses of General Electric company into a 1,500-pound parcel only three feet in diameter, four feet long. It is relatively as easy to roll in position as the X-ray your dentist pulls up to the chair for a peek through your teeth. It draws only four kilowatts of electric power. Small in size, modest in consumption of current, it is ravenous in appetitefor work. Because of its enormous power, it can be focused on its subject from a greater distance, thus cover a wider range of vision. Result: where it took six exposures to X-ray a heavy casting for a bombing plane at the Ford Motor plant with a 400,000-volt machine, the million-volt X-ray takes in six entire castings in one exposure. Where a half-inch crack in a weld four inches deep could be detected, the new machine shows up 1/8-inch cracks in an eight-inch weld. Until two months ago it took 7 1/2 hours to make an exposure of an area 12 inches wide; the same steel 24 inches wide can now be X-rayed in 15 minutes. In short, the big X-ray can do the work of millions of dollars worth of radium - more than is available in the world. In the General Electric plant, where turbines for the navy and other important defense machines are built, the big X-ray is used to take stereoscopic views, two pictures made from different angles giving a three-dimensional look through the wall of a large casting. Thus the exact location of any possible defect is made apparent. Development of this million-volt instrument was made possible by thrée important innovations. One was the use of Freon gas under pressure as an insulating medium. At first air was the insulator, then oil; 100 pounds of Freon performs the insulating task of 12,000 pounds of oil. The second development was the “resonance” transformer. Ordinary transformers consist of two coils of wire wound on an iron core. In the resonance transformer the iron core is omitted; the secondary - the coils from which high voltage emanates - has several hundred thousand turns, giving it high inductance. Without the iron core, insulation problems are simplified and the transformer’s tank diameter is one-third less. Furthermore, it can be operated in any position, giving the tube greater flexibility and mobility. Third innovation is the tube itself, which consists of twelve sealed-off sections. Electrons producing the rays emanate from a heated filament at the top, and as they flow through each section they are given a fresh jolt of about 84,000 volts. By the time they hit the bottom section of the tube they have accumulated a million volts. Then they hit the tungsten target in an extension of ‘the fube projecting from the bottom of the tank, and produce X-rays. Most of the rays pass through the target and proceed in the direction the tube is pointed, so that ordinarily the tube is aimed like a gun at its object. Nearly 20 years ago industry first put X-rays to work, when apparatus was installed in the government’s Watertown Arsenal. In the years since it has played a vital part in the onward march of aviation, helping aircraft builders adapt aluminum and other light metals to engines and structural parts requiring lightness with strength. It made possible the substitution of welding for riveting in high-pressure boilers and other heavy products by serving as the welder’s “second sight,” enabling him to lay down welds stronger than the parent metal. Now the million-volt X-ray has put on its overalls in the aircraft plants of the Ford Motor company, the boiler works of Babcock & Wilcox, the Combustion Engineering company factory, and in its own home plant of General Electric at Schenectady. It is going to work soon at the American Steel Foundries and the Campbell, Wyant and Cannon Foundry works, and, focusing its appraising eye on big guns and battleship armor at the Norfolk and Philadelphia Navy Yards, will play a great part in making America’s defense invincible.