worm, thus obstructing the shaft and leaving the type-wheels at the
zero-point until released by the printing lever. This device is too
well known to require a further description. It is not applicable to any
instrument using two independently moving type-wheels; but on nearly if
not all other instruments will be found in use." The stock ticker has
enjoyed the devotion of many brilliant inventors--G. M. Phelps, H. Van
Hoevenbergh, A. A. Knudson, G. B. Scott, S. D. Field, John Burry--and
remains in extensive use as an appliance for which no substitute or
competitor has been found. In New York the two great stock exchanges
have deemed it necessary to own and operate a stock-ticker service for
the sole benefit of their members; and down to the present moment the
process of improvement has gone on, impelled by the increasing volume of
business to be reported. It is significant of Edison's work, now dimmed
and overlaid by later advances, that at the very outset he recognized
the vital importance of interchangeability in the construction of this
delicate and sensitive apparatus. But the difficulties of these early
days were almost insurmountable. Mr. R. W. Pope says of the "Universal"
machines that they were simple and substantial and generally
satisfactory, but adds: "These instruments were supposed to have been
made with interchangeable parts; but as a matter of fact the instances
in which these parts would fit were very few. The instruction-book
prepared for the use of inspectors stated that 'The parts should not be
tinkered nor bent, as they are accurately made and interchangeable.' The
difficulties encountered in fitting them properly doubtless gave rise
to a story that Mr. Edison had stated that there were three degrees of
interchangeability. This was interpreted to mean: First, the parts will
fit; second, they will almost fit; third, they do not fit, and can't be
made to fit."

[Footnote 2: This I invented as well.--T. A. E.]