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That makes sense, but seems like an oversimplification.  The particles
aren't going N S N S N S.  I'd say it's more like
nnnnNnnnsssSsss.
Looking at it that way, I don't see how it could reverse polarity.

- E


On Tue, Mar 10, 2015 at 6:29 PM, John Chester <[log in to unmask]> wrote:

> On 3/10/15 4:14 PM, Ellis Burman wrote:
>
> re: playing backwards flips polarity
>
>> I too have been trying to wrap my head around that one.  Isn't it
>> analogous
>> to playing a record backwards?  The stylus still moves in the same
>> direction at the same point in the groove.
>>
>> If someone can explain it, I'd love to hear it.  It would be super easy to
>> verify using a tape machine.
>>
> The repro head of an audio tape machine is not sensitive to absolute
> magnetic flux level.  You can put as strong a magnet as you like against
> the head, and if the magnet is not moving, there's no output from the
> head.  Move the magnet one way, head has positive output. Move the magnet
> the other way, head has negative output.
>
> Slightly more technical explanation:
> The head is sensitive to the rate and direction of change of magnetic
> flux.  Consider a recording which leaves alternating south and north poles
> on the tape, represented here by S and N:
>
>    S N S N S N S N
>
> If we play this recording forward (which, let's say, is moving left to
> right across this diagram) then the initial transition is S to N.  Let's
> say we have connected the repro head so that this produces positive output
> voltage.  Peak output voltage will occur halfway between S and N, because
> that's the point where flux change is most rapid.
>
> If we play this recording backward, the flux change at the left end of the
> recording is N to S, and this produces a negative output voltage.
>
> Thus, playing the tape backward does flip the polarity.  The same point on
> the tape (in this example the point between the leftmost S and N) produces
> a positive voltage when played one way, and a negative voltage when played
> the other way.
>
> -- John Chester
>



-- 
Ellis
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