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Babbage's last laugh

Sep 9th 1999

From The Economist print edition

Mechanical computing devices sound like a throwback to the 19th century, but could have a bright future.

WHEN the Science Museum finished building Charles Babbage's Difference Engine No 2 in 1991, more than 150 years after its invention, mechanical computation seemed like the ultimate anachronism. Using gears and levers driven by a hand-crank to evaluate seventh-order polynomial equations to 31 significant figures is impressive as an example of Victorian engineering. But it is hard to imagine a less futuristic contraption.

Mechanical computation could, however, be on the verge of an unlikely resurgence, albeit with a 21st-century twist, as a result of research in the field of micro-electro-mechanical systems (MEMS). Scientists in both Britain and America have developed mechanical chips with moving parts that are capable of functioning as logic circuits by switching a current on and off mechanically, rather than electronically.

Such research may seem fanciful, given the ubiquity of electronic computers, which use microscopic transistors, etched on to silicon, to do the same thing. But mechanical computers would be able to withstand extremes of temperature and radiation, making them ideal for use in hostile environments such as on satellites and space probes, or in safety systems for nuclear power plants. Indeed, in some situations, mechanical circuitry would have clear advantages over the electronic kind. Suddenly, mechanical computing may not be quite such an outdated idea after all.

Electronic chips consist of millions of transistors, each of which does one simple thing: it uses the voltage at one of its three junctions to decide whether or not to let current flow between the other two. Such gatekeeping depends on being able to keep the energy of the electrons dancing around inside the device at specific levels. When the transistor switches, the change in voltage alters the electronic properties of the material and determines whether or not electrons can flow through it. It is akin to using tiny adjustments to the height of a dam to control a torrent of water. The problem is mat the "water level" (the electron energy) has to be kept within certain strictly defined limits. If it is too high, the water spills over even when the dam is up. The current then flows when it is not supposed to, and the wrong signal (a one instead of a zero) is received at the other end.

That is what happens when a computer gets too hot, or is bombarded with certain kinds of radiation: the electrons pick up enough energy to breach the dam that has been carefully constructed to contain them. As a result, electronic computers behave erratically when operated outside their recommended (and usually narrow) temperature range, or in the presence of strong radiation.

MEMS switches, on the other hand, work in the most obvious of ways: by making and breaking an electrical connection physically, with a small conducting arm, like a Morse-telegraph key. When the connection is broken, current cannot flow without travelling through the air (a spark flying). This is a barrier of a higher order, and is enough to make the micro-mechanical switch dramatically more reliable in extreme conditions.

Researchers at the University of California, Berkeley, and the Defence Evaluation & Research Agency (DERA) in Malvern, England, say the technology is practical for other reasons too. Engineers use roughly the same technique to make MEMS structures as they do to make conventional microchips: by etching tiny feature on to silicon wafers.

Admittedly, there are deviations from the procedures of electronics manufacturing, such as the need to release the tiny gears, flaps and springs so that they can move freely. But MEMS me becoming easier and cheaper to manufacture. They are already being used in car-airbag sensors and digital image projectors. Micro-mechanical logic chips could eventually be as small and easy to mass-produce as conventional ones.

lever - рычаг

gear - механизм

hand-crank - заводная (пусковая) рукоятка

contraption - новое хитроумное изобретение

verge - грань

resurgence - возрождение

albeit - хотя

twist - изгиб, поворот

ubiquity - вездесущность; повсеместность

etched - вытравленный; травлёный

withstand - устоять

hostile - враждебный

outdated - старомодный

junction - контакт

spill - проливать

dam - затвор

torrent - поток

breach - пробивать

behavу - вести себя

narrow - узкий

presence - присутствие; наличие

obvious - очевидный

dramatically - эффектно, ярко, заметно, впечатляюще

reliable - надёжный

roughly - грубо, резко

conventional - стандартный; удовлетворяющий техническим условиям

wafer - плата

admittedly - общеизвестно, по общему признанию

deviation - отклонение

flap - створка, заслонка