Re: springs or wire coils?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi RQ, All!

>instead of doing tedious winding for solenoids and chokes, can
relatively
>long (like 1ft.) springs be used? and, if so,  if a ferrite core is
needed,
>can a relatively long nail be pushed into the sprinng?
>
>spring winding~~~~~~~~~~~~~~~~~~~~~~~~~~~
>I____________________________________
>I___ferrite_nail________________________>
>Ispring winding~~~~~~~~~~~~~~~~~~~~~~~~~~~


Interesting idea.  It could be useful where you wanted to tap the
solenoid/choke at various points.  You'd have to be very careful not
to get shorted turns though, that could really spoil your day.  There
will be relatively few turns on the average spring, so the inductance
available is likely to be limited to (sub-)millihenries, which will be
a major restriction for power frequency applications.

Steel springs are *theoretically* out for rf use (just in case you
were thinking of using a big slinky as a TC :-) because the skin
effect losses in magnetic materials are very high; however, I have to
note that the ARRL successfully carried out tests using big slinkies
as ham aerials on the low frequency bands - but I think that was for
receive only where power loss wasn't a consideration.  Secondary coil
losses are a lot less important than once thought to be for
spark-driven TCs, so you might get away with it even so.

Your question re ferrite cores makes me think that you are indeed
considering the use of springs at rf (a core made from e.g. annealed
iron wire or cut-up transformer laminations would be better at power
frequencies).  Well, if you could find a non-ferrous spring, e.g. a
phosphor bronze spring, it might do without too much rf loss, but
these things are likely to be scarce and expensive, also unlikely to
be very large (unless you're into salvaging WW2 magnetic mines ;-)
Any ferrite core used should fill a significant proportion of the
spring cross-section to be effective.  Also be aware that some
ferrites are of low resistivity and that high voltage on the spring
may short through the ferrite if you don't provide enough insulation.
If you want to try building a ferrite-cored TC prepare for Trouble,
you will end up with a very nasty pulse transformer - fine if that's
what you're after!

Dunckx
Geek#1113 (G-1)