In the 19th century, British mathematician Charles Babbage invented the "difference engine," a mechanical computer that had an enormously complex arrangement of levers, ratchets and gears. Had this prototypical chunk of steampunk machinery ever been completely built, it would have weighed several tons.

Now, mechanical computers are back. Teams of scientists across the US are building new versions, though they're a bit smaller. In fact, the moving parts in these "nanomechanical" computers may eventually be smaller than the smallest silicon transistors.

The research is being funded by the US Defense Advanced Research Projects Agency, which hopes to develop computers that can survive in hot spots where conventional semiconductors would fry, such as inside weapons. Researchers say that the tiny mechanical computers will be far more energy-efficient than traditional semiconductors, will produce less heat and will withstand big voltage spikes that can burn out ordinary processors.

Some of the companies that are racing to get the next round of DARPA funding foresee myriad civilian applications for the tiny mechanical computers.

At GE Global Research, "there may be applications across multiple GE businesses, but it's not crystallized yet," says researcher Kanakasabapathi Subramanian.

That's because the technology is far from proven. GE and other companies and universities with DARPA funding are just beginning to show that physical devices can be made to move and do useful things in spaces comparable to those occupied by today's silicon nanocircuits. GE has used a "bottom-up self-assembly" method to grow nanowires from a silicon base. The hope is to make the wires act as switches, the fundamental building blocks of computers. Combining the nanowires with conventional circuit etching by lithography would result in a kind of electromechanical relay switch.

"Mechanical switches offer some unique advantages over solid-state electronic switches like transistors," Subramanian says. "One is the ability to minimize the heat that's generated within the system. Because you have a physical air gap, you minimize leakage currents that give you heat. When the switch is off, it's really off."

Having shown that it can make the nanowires move, GE is now attempting to make a working three-terminal switch. The eventual goal at GE and elsewhere is to combine the switches in various ways to build logic gates. Some researchers hope to build nanomechanical memories as well.

Researchers at the University of Wisconsin in the US have built a transistor out of "nanomechanical pillars." The pillars are made of silicon oxide tipped with gold, and the smallest are 30 nanometers at the base -- smaller than the circuit features in today's silicon chips. Robert Blick, a professor of electrical and computer engineering, says he has made the pillars act as physical switches that he calls NEMSETs, for nanoelectromechanical single-electron transistors. He says he hopes to someday build the pillars using diamond or sapphire, which are much more thermally stable than silicon. "We can go down to 10nm with current technology," he adds.

While nanomechanical devices have several advantages over semiconductors, they are unlikely to ever match the performance of silicon-based processors, Blick says, since they are limited to clock speeds of about 2 GHz. But, he notes, the mechanical parts can be built on top of a silicon substrate so that performance-demanding functions could be carried out on conventional CMOS coupled to the mechanical computer.

The next step, Blick says, is to build simple logic devices, such as XOR and NAND gates, and combine them into complex logic circuits. But that's still only the beginning. A single logic device must then be made to drive a number of other devices in a chain that doesn't lose voltage, a process known as "fanout." Finally, ways to manufacture them cheaply and reliably must be found.