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inductor question

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

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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.

Reply to
R.Spinks
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Where does your formula come from? The closest reference I could find was this page:

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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 inductance.

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John Popelish
Reply to
John Popelish

stretch

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Reply to
R.Spinks

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