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From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: HELP!
Message-ID: <17660144@hpfcdj.HP.COM>
Date: 24 Jan 91 19:13:07 GMT
References: <1991Jan19.052458.7449@wam.umd.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 55

>The first electric chair was used in New York state. At the time there were
>competeing electric distribution systems AC (George Westinghouse) and
>DC (Thomas Edison). Neither company wanted to discredit their system
>by supplying the equipment for the executon device. Edison lobbied hard
>for the use of the AC system and not his DC system. Westinghouse
>refused to supply the AC system even though New York had selected it.
>Eventually New York was able to obtained used Westinghouse AC
>equipment and performed the execution.
>
>AC kills.

That's not quite the whole story, Tom.

Yes, Edison "lobbied hard" to discredit Westinghouse's AC system, and even
went so far as to set up travelling "road shows" in which small animals
were killed by applying AC, thereby "proving" that is was AC that was
lethal, while his DC system was "perfectly safe."  At the time that New York
was making its chosen form of execution electrocution, Edison's efforts
resulted in the new law being worded so as to require execution by means of
"alternating electrical currents applied to the body" (quoted from memory,
and probably not exact).  Unfortunately, no details were provided beyond
that, and when the "electric chair" system was built, the designers did NOT
guarantee a sufficiently high current through the body to cause instantaneous
death.  They had made sure that the "alternating currents" were applied, but
through the use of a big DPDT switch!

When it came time to dispose of said axe murderer, the flaw in the system
was apparent: rather than simply zapping the criminal to the Great Beyond,
he was slowly fried; execution took several minutes, as the horrified witnesses
looked on.

It is NOT simply the fact that the current is "alternating" that makes it
lethal; as has been mentioned, it is the level of *current* - esp. that current
which passes directly through the heart - which determines whether the subject
will live or die.  Voltage comes into the picture only via Ohm's Law, 
depending on the path in question.  Contact via dry skin is actually pretty
poor, and so a higher voltage is required to kill.  The critical level, as
I recall, is something on the order of 20 uA (yes, that's MICROamps) directly
through the heart - anything over this can possibly cause fibrillation. Note
that achieving this level *through the heart* can require orders of magnitude
more current through, say, the limbs making contact.  But also note that
contact made directly to wet tissue - as in direct defibrillation of the
heart, with the "little paddles" you may have seen in films of open-heart
surgery - requires very little voltage/current.

AC does have one factor which may make it more dangerous in some situations
than an equivalent level of DC - the tendency of AC (over a certain range)
to cause the subject to "freeze" in contact with the source, where DC would
be more likely to cause an involuntary "kick away."  But EITHER can kill
if sufficient current can be made to pass through the body.


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.