Unless I'm completely misunderstanding something, what Jim says here isn't
   right, and Janice is right to be concerned about polarity. Reversing
   polarity is a good way to fry things, last I heard, and not all devices use
   the same polarity. Best to be certain you've got it right before you plug
   anything in.

   Beverly


Just a footnote to Beverly's observation:

When I was a little boy, many years ago (I'm a bigger boy now), I was with my
dad, buying an oil filter at Canadian Tire.I remember asking him why there were
so many different sizes and shapes of oil filter; couldn't three or four sizes
work for every car and truck? Dad said that was a Good Question. I 
was too young
to know better than to ask such questions, knowing nothing about profits and
marketing and those sort of things.

The "standard" power plugs that we see on everything, in anything but standard
sizes (last time I checked there were NINE different sizes for these things,
hardly what I'd consider a standard), or non-standard in another very important
way: their polarity seems to be an entirely random affair, presumably based on
the whim of the engineer designing each product. While phono plugs 
are typically
positive at the tip, and RCA plugs must be positive at the center in 
order for the shield to be outside, power plugs can go either way, 
and do.

Polarity, particularly with battery packs and power supplies for 
electronics, is
most important, and you can fry your Powerbook beyond practical hope of repair
by applying reverse power to it's power input jack. Some wise guy 
will tell me he
did it once for a few seconds and got away with it, but I hear you 
can be struck
by lightning and get away with it, too. Polarity is easy to check 
with any meter,
or VOM, or even an LED, and it's very important. Particularly when 
you never know
how any given Power Plug is wired.

So there.

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