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National Advisory Committee for Aeronautics, Report - Tensile Properties of Typical Stainless Steels and Nonferrous Metals as Affected by Plastic Deformation and by Heat Treatment

During tests subsequent to those from which this
report was prepared, it was found that a constant per-
centage error had been introduced into strain readings
made with the particular Tuckerman extensometer
used, because its collimator had been equipped with
an objective lens of focal length differing from the
standard value for this instrument. An investigation
has disclosed that this collimator was a very early
type. At the time of its manufacture (1923), the ob—
jective lens was not ground 'to a fixed focal length,
but the collimator had been equipped with an auxiliary
“focal adjuster” lens, in order to make the instrument
direct reading. This auxiliary lens had long since been
discarded from the collimator, due to certain diffi-
culties in its adjustment. This fact was not known to
the authors until after the report was in print.

During tests subsequent to those from which this
report was prepared, it was found that a constant per-
centage error had been introduced into strain readings
made with the particular Tuckerman extensometer
used, because its collimator had been equipped with
an objective lens of focal length differing from the
standard value for this instrument. An investigation
has disclosed that this collimator was a very early
type. At the time of its manufacture (1923), the ob—
jective lens was not ground 'to a fixed focal length,
but the collimator had been equipped with an auxiliary
“focal adjuster” lens, in order to make the instrument
direct reading. This auxiliary lens had long since been
discarded from the collimator, due to certain diffi-
culties in its adjustment. This fact was not known to
the authors until after the report was in print.