At present, commercial stun guns target one person at a time, and work only at close quarters. The new breed of non-lethal weapons can be used on many people at once and operate over far greater distances.

But human rights groups are appalled by the fact that no independent safety tests have been carried out, and by their potential for indiscriminate use.

The weapons are designed to address the perceived shortcomings of the Taser, the electric-shock gun already used by 4000 police departments in the US and undergoing trials with some police forces in the UK.

It hits the victim with two darts that trail current-carrying wires, which limit its range to a maximum of 7 metres. As a single shot, short-range weapon, the Taser is of little use in crowd control. And Tasers have no effect on vehicles.

These limitations are beginning to be overcome. Engineers working for the US Department of Defense's research division, DARPA, and defence companies in Europe have been working out how to create an electrically conductive path between a gun and a target without using wires.

A weapon under development by Rheinmetall, based in D�sseldorf, Germany, creates a conducting channel by using a small explosive charge to squirt a stream of tiny conductive fibres through the air at the victim.

Meanwhile, Xtreme Alternative Defense Systems (XADS), based in Anderson, Indiana, will be one of the first companies to market another type of wireless weapon. Instead of using fibres, the $9000 Close Quarters Shock Rifle projects an ionised gas, or plasma, towards the target, producing a conducting channel. It will also interfere with electronic ignition systems and stop vehicles.

"We will be able to fire a stream of electricity like water out of a hose at one or many targets in a single sweep," claims XADS president Peter Bitar.

The gun has been designed for the US Marine Corps to use for crowd control and security purposes and is due out next year. It is based on early, unwieldy technology and has a range of only 3 metres, but an operator can debilitate multiple targets by sweeping it across them for "as long as there is an input power source," says Bitar.

XADS is also planning a more advanced weapon which it hopes will have a range of 100 metres or more. Instead of firing ionised gas, it will probably use a powerful laser to ionise the air itself. The idea has been around for decades, says LaVerne Schlie, a laser expert at the US Air Force Research Lab in Kirtland, New Mexico. It has only become practical with advances in high-power solid-state lasers.

"Before, it took a laser about the size of two trucks," says Schlie. "Now we can do it with something that fits on a tabletop."

The laser pulse must be very intense, but can be brief. So the makers of the weapons plan to use a UV laser to fire a 5-joule pulse lasting just 0.4 picoseconds- equating to a momentary power of more than 10 million megawatts.

This intense pulse - which is said not to harm the eyes - ionises the air, producing long, thread-like filaments of glowing plasma that can be sustained by repeating the pulse every few milliseconds. This plasma channel is then used to deliver a shock to the victims similar to a Taser's 50,000-volt, 26-watt shock.

HSV Technologies of San Diego, California is also working on stun and vehicle-stopping shock weapons with ranges of over 100 metres. And another company, Ionatron of Tuscon, Arizona, is due to supply a prototype wireless vehicle-mounted weapon to the US Department of Defense by the end of the year.

But the advent of wireless stun weapons has horrified human rights groups. Robin Coupland of the Red Cross says they risk becoming a new instrument of torture. And Brian Wood of Amnesty International says the long-range stun guns could "inflict pain and other suffering on innocent bystanders".

And there are safety concerns. Of the 30,000 times US police officers have fired Tasers, in 40 instances people stunned by them later died. The deaths have been attributed to factors such as overdoses of drugs and alcohol, or fighting with officers, rather than the electric shock.

In a statement, Taser International chief Rick Smith said: "In every single case the medical examiner has attributed the direct cause of death to causes other than the Taser." Amnesty is not convinced, however, and wants an independent study of the effects of all existing and emerging electric-shock weapons.

This article will appeared appear in the June 19 edition of New Scientist

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