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inductor question

Discussion in 'Electronic Basics' started by R.Spinks, Sep 19, 2004.

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  1. R.Spinks

    R.Spinks Guest

    If you have a length (l) of wire coiled to make an inductor and you stretch
    the wire (ie. space the coils further) but do not change the diameter (see is the inductance the same? I
    understand inductance to be L = (mu N^2 A) / l + 0.45d. So if I'm stretching
    the coils but not changing l
    (length) will the inductance change? I guess that technically the cross
    sectional area (A) would be changing slightly due to stretching to coils ...
    but I'm not sure if that minor variation is negligible - since I'm not
    actually changing the diameter. I thought that you compress/decompress
    coils (which is what I think this example is) to tune inductance... is this
    what's happening? How does a variable inductor work? I have no equipment so
    I can't just coil some wire and stretch it myself to see. Insight is
    appreciated. Thanks.
  2. Where does your formula come from? The closest reference I could find
    was this page:

    A common formula for an air core cylinder coil is Wheeler's formula,

    L (microhenries) =(0.8 * N^2 * R^2)/(6*R + 9*L + 10*B).
    N = total number of turns
    R = average radius = (inner radius + outer radius)/2
    L = coil length (along the axis)
    B = thickness of the winding = outer radius - inner radius
    (all dimensions in inches, coil immersed in air)

    The L in that equation does not refer to the length of the wire, but
    to the length of the cylinder of the coil's form. So stretching the
    coil out into a longer cylinder certainly changes L and thus the
  3. R.Spinks

    R.Spinks Guest

    I got it from "Introduction to Electronics" by Dorf. 2nd Ed. pg. 247 (old
    college book)
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