IEEE Spectrum December, 2007 - 19
www.spectrum.ieee.org
10 000
Current, amperes
Lightning
WHAT A JOLT: The mild
shock from a wool sweater
may carry almost as much
current as the defibrillator
that jolts an errant heart back
into action. The effect of a
shock on the human body,
however, varies greatly with
pulse duration and current.
1000
100
Automated
external
defibrillator
10
Static shock
Taser
(single
pulse)
1
0.0001
0.001
0.01
0.1
1
10
100
Pulse duration, milliseconds
10 000
Leads to internal burns
1000
Causes heart to
behave erratically
Current, milliamperes
second, less than a fifth the speed of a bullet from a typical pistol.
Each projectile, which weighs 1.6 grams, has a 9-millimeter-long
tip to penetrate clothing and the insulating outer layer of skin.
Two whisper-thin wires trail behind for up to 9 meters, forming
an electrical connection to the gun.
Because the barbs get stuck in clothing and fail to reach the
skin about 30 percent of the time, the gun is designed to generate
a brief arcing pulse, which ionizes the intervening air to establish a conductive path for the electricity. The arcing phase has
an open-circuit peak voltage of 50 000 volts; that is, the voltage
is 50 kilovolts only until the arc appears or until the barbs make
contact with conductive flesh, which in the worst conditions offers
around 400 ohms of resistance [see illustration, "Freeze!"].
The target's body is never exposed to the 50 kV. The X26-
the model commonly used by police departments-delivers a peak
voltage of 1200 V to the body. Once the barbs establish a circuit,
the gun generates a series of 100-microsecond pulses at a rate of
19 per second. Each pulse carries 100 microcoulombs of charge, so
the average current is 1.9 milliamperes. To force the muscles to
contract without risking electrocution, the signal was designed to
exploit the difference between heart muscle and skeletal muscle.
Skeletal muscle constitutes 40 percent of a typical person's
mass and is responsible for making your biceps flex, your fingers type, and your eyelids wink. It's organized into bundles
of single-cell fibers that stretch from tendons attached to your
skeleton. When your brain orders a muscle to flex, an electrical
impulse shoots down a motor nerve to its termination at the
midpoint of a muscle fiber. There the electrical signal changes
into a chemical one, and the nerve ending sprays a molecular
transmitter, acetylcholine, onto the muscle. In the milliseconds
before enzymes have a chance to chew it up, some of the acetylcholine binds with receptors, called gated-ion channels, on the
surface of the muscle cell. When acetylcholine sticks to them,
they open, allowing the sodium ions in the surrounding salty
fluid to rush in.
The movement of those ions raises the cell's internal voltage,
opening nearby ion channels that are triggered by voltage instead
of by acetylcholine. As a result, a wave of voltage rolls outward
along the fiber toward both ends of the muscle, moving as fast as
5 meters per second. As the voltage pulse spreads, it kick-starts
the molecular machinery that contracts the muscle fiber.
By directly jolting the motor nerves with electricity, a Taser
can stimulate the muscle and get the same effect.
The force with which a skeletal muscle contracts depends on
the frequency at which its nerve fires. The amount of contraction elicited is proportional to the stimulation rate, up to about
70 pulses per second. At that point, called tetanus, contractions
can be dangerously strong. (The same thing happens in the disease tetanus, whose primary symptom, caused by the presence
of a neurotoxin, is prolonged contraction of skeletal fibers.) The
Taser, with its 19 pulses per second, operates far enough from
the tetanus region so that the muscles contract continuously but
without causing any major damage.
Heart muscle has a somewhat different physical and electrical
structure. Instead of one long cell forming a fiber that stretches
from tendon to tendon, heart muscle is composed of interconnected
fibers made up of many cells. The cell-to-cell connections have
a low resistance, so if an electrical impulse causes one heart cell
to contract, its neighbors will quickly follow suit. With the help
of some specialized conduction tissue, this arrangement makes
the four chambers of the heart beat in harmony and pump blood
efficiently. A big jolt of current at the right frequency can turn
the coordinated pump into a quivering mass of muscle. That's
100
Interferes with breathing
Locks up muscles
10
Taser shock
Induces
pain
1
Causes tingling sensation
0.1
10
100
1 000
10 000
Frequency, hertz
LEVELS OF SHOCK: The Taser X26 puts out 2 milliamperes at 19 hertz. The gun
packs its current into 100-microsecond pulses, so it can capture muscle with lower
current than if it had been delivered as a sine wave, as the rest of the chart shows.
just what electrocution does: the burst of electricity causes the
heart's electrical activity to become chaotic, and it stops pumping
adequately-a situation known as ventricular fibrillation.
The Taser takes advantage of two natural protections against
electrocution that arise from the difference between skeletal and
cardiac muscle. The first-anatomy-is so obvious that it is typically overlooked. The skeletal muscles are on the outer shell of the
body; the heart is nestled farther inside. In your upper body, the
skeletal muscles are arranged in bands surrounding your rib cage.
Because of skeletal muscle fibers' natural inclination to conduct lowfrequency electricity along their length, a larger current injected
into such a muscle tends to follow the grain around the chest rather
than the smaller current that penetrates toward the heart.
The second protection results from the different timing
requirements of the nerves that trigger muscle contractions and
the heart's intrinsic electronics. To lock up skeletal muscle with-
December 2007 | IEEE Spectrum | NA 29
http://www.spectrum.ieee.org
Table of Contents for the Digital Edition of IEEE Spectrum December, 2007
IEEE Spectrum December, 2007 - Cover1
IEEE Spectrum December, 2007 - Cover2
IEEE Spectrum December, 2007 - 1
IEEE Spectrum December, 2007 - 2
IEEE Spectrum December, 2007 - 3
IEEE Spectrum December, 2007 - 4
IEEE Spectrum December, 2007 - 5
IEEE Spectrum December, 2007 - 6
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IEEE Spectrum December, 2007 - 16
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IEEE Spectrum December, 2007 - 28
IEEE Spectrum December, 2007 - 29
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IEEE Spectrum December, 2007 - 33
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IEEE Spectrum December, 2007 - 47
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IEEE Spectrum December, 2007 - 50
IEEE Spectrum December, 2007 - 51
IEEE Spectrum December, 2007 - 52
IEEE Spectrum December, 2007 - Cover3
IEEE Spectrum December, 2007 - Cover4
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