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Matthias, F. T. (ed.) / The Wisconsin engineer
Volume 33, Number V (February 1929)

Wegel, R. L.
Mechanical delay-network,   pp. 163-164

Page 163

The Mechanical Delay 'Network
                                     By R. L. WEGEL*
                          Bell Telephone Laboratories, New  York
SOMEWHAT reminiscent of the string telephones of
our youth is an entertaining off-shoot of Bell Tele-
phone Laboratories' research in acoustics, the recently
developed mechanical delay-network. Essentially the device
is a helical spring, hanging loosely between a transmitting
and a receiving element. Wandering through the con-
necting wire coil, sound vibrations impressed at one end
appear at the other some time later.
  When recently shown by Mr. S. P. Grace, Assistant
Vice President of the Laboratories, to the regional meeting
of the American Institute of Electrical Engineers in At-
lanta, the annual convention of Telephone Pioneers in
Boston and the Cleveland Engineering Society, the delay-
spring was incorporated in a demonstration apparatus.
From a microphone transmitter Mr. Grace's voice proceeded
to the spring, whose output, translated into electrical vi-
brations and amplified, was reproduced by a loud-speaker.
The audience thus heard the same words twice: directly
from Mr. Grace's mouth and one second later from the
   Due to the physical complexity of even their simplest
forms, the theoretical explanation of this sort of apparatus
is at best difficult. It is not always safe to make simplify-
ing assumptions in discussing these problems; questions of
speech transmission especially must be treated in consid-
erable detail, for many of the most curious and important
properties of mechanical structures vibrating at voice-fre-
quencies result precisely from this complexity. Fortunately
much assistance can be derived from the use of the familiar
analogy between electrical and mechanical vibratory sys-
tems. Reference to the analogy, even if only qualitative,
often makes it possible to predict from electrodynamic
experience the nature of the effects which may be expected
of the analogous mechanical apparatus.
   Treating in this way, the famous problem of beads on a
string illustrates well the aspects of vibrating systems which
occasion their apparently strange behavior. And, simpler
than that of the delay-spring, it is yet sufficiently similar
to make possible some significant comparisons between
   In the idealized form of this classical problem, first dis-
cussed by Lagrange and recently interpreted in terms of
its electrical counterpart, the string is supposed to have
negligible weight and the beads, arranged equidistantly in
   *Editor's Note: Mr. Wegel, a 1910 graduate of Ripon College,
acted as assistant in Physics at the University of Wisconsin from
1910 to 1912. After a year as physicist with Thomas A. Edison,
he joined Bell Telephone Laboratories in 1914, where he has
made important contributions to the design and theory of eiectro
mechanical vibratory apparatus. This article is taken from  the
Bell Telephone Quarterly.
succession along it, to be infinitesimally small. The result-
ing model is a series of masses, concentrated at mathematical
points, connected by pure weightless elasticity, and vibrat-
ing transversely to the direction of the string. Its electrical
analogy is a low-pass wave filter, with series inductances
The Transmitter End of the Network. Spring is Shown at "A".
corresponding to the masses and shunt capacities to the
segments of the connecting elastic string.  More generally
the mass-point could, of course, vibrate in three inde-
pendent directions, the three mutually perpendicular co-
ordinates of space, two of them transverse to the direction
of the string and one along it. The corresponding analogy
is three low-pass filters, through any of which impulses
could be independently propagated.
  When, however, beads are actually placed on a string,
the ideal condition cannot ordinarily be realized sufficiently
closely to permit its assumption in simplifying the study of
voice transmission over the string. The beads must have
length, breadth and thickness, the string must have ap-
preciable weight, and the beads cannot be exactly centered
on the string.
  Introducing these complicating realities gradually, it is
simplest to consider first beads on an ideal string: a
weightless string, bearing beads possessed of dimensions and
mounted off their centers of mass. It is apparent that a
force applied through the elastic string to one of the beads
at a point not on its centre of mass will tend not only to
move the bead in the direction of the force but to rotate
it as well.  The beads may travel in circles or ellipses
rather than in a straight line along the direction of the
drive. Each bead must, therefore, be considered as an
extended body, requiring six coordinates to specify its
position or velocity at any particular time: the three co-

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