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
Looking at it that way, I don't see how it could reverse polarity.
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
>> 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
[log in to unmask]