Academic Reading — Test 169

Reading passage

In the years before the Second World War, the British government grew increasingly anxious about the threat of aerial attack. Engineers and scientists were asked to find a way of detecting approaching aircraft long before they could be seen or heard. The solution that emerged was a system based on radio waves, later known as radar. By transmitting pulses of radio energy and listening for the faint echoes that bounced back from distant objects, operators could estimate the position and distance of an aircraft. The first British network, built along the southern and eastern coasts, used relatively long radio wavelengths and required enormous towers to broadcast its signals. Although this early system proved its worth during the air battles of 1940, it had a serious limitation: the long wavelengths produced beams that were broad and imprecise, making it difficult to pinpoint a target with accuracy. The obvious remedy was to use much shorter wavelengths, in the region known as microwaves. Shorter waves could be focused into a narrow beam by a comparatively small aerial, which meant that compact equipment might one day be carried aboard aircraft and ships. The difficulty was that no existing device could generate microwaves at a power high enough to be useful. The valves available at the time produced only feeble signals when pushed to such high frequencies, and without a strong transmission the returning echoes would be too weak to detect. For several years this technical barrier held back the whole field, and many researchers doubted that a practical solution would be found in time. The breakthrough came in early 1940 at the University of Birmingham, where two physicists, John Randall and Harry Boot, were working on the problem. Drawing on earlier ideas about how electrons behave inside magnetic fields, they designed a new kind of valve which they called the cavity magnetron. It consisted of a solid block of copper into which a ring of small circular chambers, or cavities, had been drilled. A magnet forced a stream of electrons to travel in curved paths past the mouths of these cavities, and as they did so they set up powerful oscillations, rather as blowing across the neck of a bottle produces a note. The device was remarkably small, yet it generated microwave power thousands of times greater than anything achieved before. Crucially, it could be manufactured fairly cheaply once the design was settled. The military significance of the cavity magnetron was immediately apparent. Compact radar sets built around it could be installed in night fighters, allowing pilots to locate enemy bombers in darkness with a precision that had previously been impossible. Fitted to patrol aircraft, the same technology helped crews to spot the conning towers of submarines on the surface of the sea, which contributed greatly to the protection of vital supply convoys. On the ground, accurate microwave radar improved the aiming of anti-aircraft guns. Yet Britain in 1940 lacked the industrial capacity to produce the new sets in the quantities the war demanded, and its factories were already under severe strain. The answer was to share the secret with the United States, which had not yet entered the war. In the autumn of 1940 a British delegation crossed the Atlantic carrying a number of technical treasures, the most important of which was an early magnetron. The decision to hand over so valuable a device was bold, but it allowed American industry, with its vast resources, to refine the design and to manufacture radar equipment on a scale that Britain alone could never have matched. Historians often describe the magnetron as the most valuable cargo ever brought to American shores, and the collaboration it began laid the foundations for a partnership in military technology that would last for decades. After the war the magnetron found peaceful uses that its inventors had not foreseen. The same ability to generate intense microwaves was applied to cooking, and the domestic microwave oven, which became common in homes across the world, descends directly from the wartime device. Civil aviation came to depend on radar for the safe guidance of aircraft, and ships of every kind navigated more safely because of it. In this way a small copper valve, designed under the pressure of war to meet a single urgent need, went on to shape ordinary life in countless ways that its hurried creators could scarcely have imagined.