ould "receive a message
off the wire", by listening to the clicking of the sounder. The number
of letters sent or received per minute was taken as the measure of his
proficiency. This number increased rapidly in the first few weeks, and
then more and more slowly, giving a typical learning curve, or
"practice curve", as it is also called.

[Illustration: Fig. 51.--(From Bryan and Harter.) Practice curve of
student W. J. R. in learning telegraphy. The height of the curve
indicates the number of letters sent or received per minute. Therefore
a rise of the curve here indicates improvement.]

The curve for sending, aside from minor irregularities, rose with a
fairly smooth sweep, tapering off finally towards the "physiological
limit", the limit of what the nerves and muscles of this individual
could perform.

[Footnote: A good example of the physiological limit is seen in the
hundred yard dash, since apparently no one, with the best of
training, can lower the record much below ten seconds; and any given
individual's limit may be considerably worse than this, according to
his build, muscular strength and quickness of nerve centers. The
simple reaction gives another good example; every one has his limit,
beyond which no amount of training will lower his reaction time; the
neuromuscular system simply will not work any faster.]

The receiving {322} curve rose more slowly than the sending curve, and
flattened out after about four months of practice, showing little
further improvement for the next two months. This was a discouraging
time for the student, for it seemed as if he could never come up to
the commercial standard. In fact, many learners drop out at this
stage. But this student persisted, and, after the long period of
little improvement, was gratified to find his curve going up rapidly
again. It went up rapidly for several months, and when it once more
tapered off into a level, he was well above the minimum standard for
regular employment.

Such a flat stretch in a practice curve, followed by a second
rise--such a period of little or no improvement, followed by rapid
improvement--is called a "plateau". Sometimes due to mere
discouragement, or to the inattention that naturally supervenes when
an act becomes easy to perform, it often has a different cause. It
may, in fact, represent a true physiological limit for the act as it
is being performed, and the subsequent rise to a higher level may
res