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Wednesday, 1 August, 2001, 13:37 GMT 14:37 UK
How the stun gun works
Taser gun
BBC News Online examines the technology behind the electric stun guns which police in London are considering as a non-lethal way of dealing with violent suspects.

The air taser gun looks like a pistol but uses compressed air to fire two darts that trail electric cable back to the handset.

When the darts strike, a five-second 50,000-volt charge is released down the cable, causing the suspect's muscles to contract uncontrollably.

Officers do favour them as a useful tool to do the job of protecting the public

Mike McBride, editor of Police and Security Equipment
The taser causes temporary paralysis and research suggests there is little permanent risk to health.

A laser helps target the suspect and the taser works at ranges up to 21ft.

Positive feedback

Electrical signals - taser waves or T-Waves -overpower the body's normal electrical signals, temporarily confusing the nervous system.

Mike McBride, editor of Jane's Police and Security Equipment, told BBC News Online that feedback from police forces in the US, where the taser guns are used, had been positive.

"They are extremely useful in dealing with certain situations where a firearm would be inappropriate, say where a suspect is armed with a broken bottle," he said.

Mr McBride, whose publication provides a guide to the latest police and security equipment available, has shadowed US police carrying the weapons.

"Officers do favour them as a useful tool to do the job of protecting the public," he said.

He has also seen the weapon on trial in Germany.

Advanced taser guns have the advantage of being able to get through thick clothing, he continued.

Most people will automatically curl up to protect themselves against the taser gun, allowing officers to move in safely.

Technologically advanced

The guns are so technologically advanced that it will be possible to determine how and when the weapon was used.

You can verify when it was used and how long it was used for. This improves accountability of police

Mike McBride

Mr McBride said every time the taser is fired it also releases up to 40 confetti-like ID tags which would identify which officer had used the weapon.

And he said the weapons have a microchip which allows data to be downloaded onto a computer.

"You can verify when it was used and how long it was used for.

"This improves the accountability of police," he said.

The taser, which runs off eight batteries, automatically checks that the equipment is fully charged.

No deaths attributed

Research seems to suggest that the health risks are small.

The advanced taser has been used by police in the US without causing death.

But according to the Guardian newspaper, a pregnant woman in the US miscarried after being hit by a taser.

This prompted health experts to warn that they may "encounter complications from the taser more often" as its use becomes more widespread.

The Los Angeles sheriff's department cut short a six-month trial after four months and issued the gun to officers in June.

Research by the University of Southern California Medical Centre suggests the taser shot produces no long-term damage.

It found there was no lasting damage to nerve or heart tissue.

Should the Metropolitan Police decide to introduce the weapons, each taser gun is likely to cost 200 and officers may only require a day's training to use them.

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