20 Jun
2004
20 Jun
'04
3:43 p.m.
A fellow technician once asked me : " If I wire
2 x 9v batteries in
parallel , will I get 4.5v or 18v ? "
"No." :-)
We also had an ex- RAF guy who used to ask for a 250v
fuse - not a
12v one, a 250v one !?
That actually makes some sense. When a fuse blows, it has to be able
to interrupt the current flowing. One important part of this is the
voltage that develops across the fuse; if it's too high, the fuse may
arc over and conduct at least somewhat anyway (plasma is a relatively
low-resistance substance).
A fuse designed to break a 250V circuit will generally, for example,
have its terminals farther apart internally than one designed to break
a 12V circuit. (Once the fuse is blown and the system quiescent, the
12V fuse may be able to withstand 250V. But (a) at the moment of
blowing, there is often arc-over present, and the gap has to be large
enough to kill that arc, which requires a larger gap than necessary to
keep it from restarting; and a fuse, like any overload protective
device, should always be overdesigned by large factors anyway.)
Who was it said that ,to earlier civilisations, high
technology would
appear to be magic ?
Clarke's Third Law. "Any sufficiently advanced technology is
indistinguishable from magic" is the phrasing I've come to know it by.
("Clarke" here is Arthur C. Clarke, the sf writer.)
Not that much earlier , it would appear.
Very true. Or like the person who, upon running out of money in her
bank account, rubbed a magnet across the magnetic stripe of her ATM
card to recharge it.
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