Forks and Temperature: the answer

[Robert Scott]

A while ago I asked if anyone had data on the exact temperature
dependancy of steel tuning forks.  Since that data was not forthcoming,
I conducted some carefully controlled tests on several different forks.
What I found was that for small temperature variations, the pitch of
the fork goes down 1 cent for every 7 degrees Fahrenheit rise in
temperature.  In particular, for A-440 forks, this translates to
one beat in 4 seconds for every 7 degrees temperature change.
Since forks are often calibrated by the manufacturer at 68 degrees,
what can you do if you have to tune a piano in August in Texas at
an open-air venue?  Well, if the air temperature is 85 degrees, this
is 17 degrees higher than the calibrated temperature of 68 degrees.
Knowing 7 degrees equals 1/4 BPS, 17 degrees is .6 BPS, or 6 beats
in 10 seconds.  So you can achieve A-440 in the piano by tuning the
A4 sharp so that it makes 6 beats in 10 seconds with the fork.

As an aside, I magnetized a fork by passing a strong magnet between
the tines. The result was a 1/2 cent rise in pitch.  A bulk tape 
demagnetizer put things back where they belong.  So make sure 
your handy picker-uper magnet is not stored next to your fork.


-Bob Scott
 Ann Arbor, Michigan