Long wires do make a difference when you consider the EMP strength is
expressed in 50,000 v/m.
The following is an excerpt about popular EMP myths:
http://www.futurescience.com/emp/EMP-myths.html
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EMP Myths
Cars dying: Some say that all vehicles traveling will come to a
halt, with all modern vehicles damaged because of their use of modern
electronics (and one movie even had a bulk, non-electronic part
dying). Most likely there will be some vehicles affected, but
probably just a small fraction of them (although this could create
traffic jams in large cities). A car does not have very long cabling
to act as antennas, and there is some protection from metallic
construction. As non-metallic materials are used more and more in the
future to decrease weight and increase fuel efficiency, this advantage
may disappear.
Wristwatch dying: One movie critic pointed out that electronics in a
helicopter were affected, but not the star's electronic watch. A
watch is much too small for HEMP to affect it.
Turn equipment off: There is truth to this recommendation (if there
were a way to know that a burst was about to happen). Equipment is
more vulnerable if it is operating, because some failure modes
involving E1 HEMP trigger the system's energy to damage itself.
However, damage can also happen, but not as easily, to systems that
are turned off.
Maximum conductor length: There is a suggestion that equipment will
be OK if all connected conductors are less than a specific length.
Certainly shorter lengths are generally better, but there is no magic
length value, with shorter always being better and longer not.
Coupling is much too complex for such a blanket statement -- instead
it should be "the shorter the better, in general". (There can be
exceptions, such as resonance effects, which depend on line lengths.)
Myth: Small transistorized radio receivers would survive a nuclear
EMP attack. Fact: In many areas affected by an EMP attack, many
small solid-state radio receivers probably would survive if their
antennas were not extended and they were not connected to any external
wires. Many other unprotected radio receivers probably would not
survive, though. Where most people go wrong is the source of the
information for their belief that radio receivers would survive. One
source for the belief is the testing of small transistorized radios
that was done during the 1970s. That testing cannot be extrapolated
to today's solid-state receivers, which usually use integrated
circuits that are much more sensitive to EMP than the receivers of the
1970s that used much more rugged discrete transistors.
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I want to argue against the ruggedness of discrete transistors used in
the 1970s. They are actually more vulnerable because they have three
metal leads coming out of the PN junctions to be soldered onto the
printed-circuited board and the induced voltage from the EMP across
those metal leads are much greater than the individual transistors
embedded inside an IC because of the physical distance between those
junction electrodes.