[CAUT] pre-stretching new string?

[Richard Brekne]

The degree any string changes tension due to a change in length to any 
of its segments is calculated by the formula given by Dr. Galembo. I've 
posted this several times now.  Again. it is 

 Change in Tension = (E*S*Change in Length) / Original length.

Where E is Youngs modulus and S is the cross section of the string.

You first figure the length change, then figure the change in tension, 
then add the change of tension to the original tension... then with both 
the new tension and new length you can figure change in pitch.  This 
works for plain wire.  If you actually apply this formula find out what 
kinds of things need to happen to a given string for it to experience 
the kind of pitch changes we observe... you immediately see that the 
pitch changes we see at the tenor bridge have no (at present) good 
explanation.

Cheers
RicB



    The degree to which a string goes out of tune will be a function of
    the break point percentage (BPP) when it is at pitch (thanks Ron N.
    for clarifying that).  The lower the BPP, the greater the change in
    pitch for a given change in length.  The top end of the bass section
    is generally considerably higher BPP than the low tenor, and the low
    tenor is usually the lowest BPP in the entire piano and thus goes
    out of tune the most with seasonal changes.

    David Love

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