Systems, Man & Cybernetics - July 2015 - 40

and AT&T by Otto Julius Zobel
(1887-1970), Ronald Martin
Foster (1896-1998), and John
Renshaw Carson (1886-1940)
as well as by Albert Charles
Bartlett, Wilhelm Cauer (1900-
1945), Hendrik Wade Bode
(1905-1982), W. Brandt, Hans
Piloty (1894-1969), Otto Walter
Heinrich Oscar Brune (1901-
1982), and Sidney Darlington
(1906-1997), who applied the
theor ies of linear algebra,
(a)
(b)
(c)
complex variables, and combinatorial topology to network
Figure 2. (a) Wiener's book [35], (b) Zadeh's report [39], and (c) the joint
synthesis and analysis. "It was
paper with Ragazzini [4].
a time when the intuition of the
electrical
engineer
needed
support from the rigorous analCampbell's studies led to the invention of the wave
ysis of the professional mathematician" [9, p. 899].
filter as circuits and a network that permit the passage
In Germany, Cauer was working on a monograph about
of signals (waves) of a particular range of frequencies,
filter circuits, which he finished in 1931 during a onewhile those with frequencies outside this range can be
year research residence in the United States with Bush's
attenuated and suppressed. Owing to its similarity to a
working group at MIT, a trip he was able to undertake
chemical process, in which large particles can be sepathanks to a Rockefeller scholarship. The book contained
rated from small ones by passing the solution through
a complete theoretical description
a membrane with pores that are
as well as listings of the various
smaller than the large particles,
filter types. In addition, it was disthis process was referred to as
tinguished by the fact that it pubelectrical filtering.
As early as 1950,
licized a new method of designing
Without going into too many of
Turing had asked
electrical wave filters that had a
the details of EE, electrical filters
whether machines
number of advantages over the
are specially adapted electrical osprinciple espoused by Zobel, who
cillating circuits, or resonators. If
can think.
had produced filters of the Campwe extend the analogy of mechanibell type. As Darlington later
cal oscillation, we can regard elecremembered:
trical filters as oscillating circuits
At Bell Laboratories, a number of us first learned
in which the current passing through a capacitor correabout Cauer's canonical circuits and his Chebyshev
sponds to the restoring force of the mechanical resonator
approximations at a conference on Cauer's proposed
and the inductivity of a coil corresponds to the inert mass.
sale of some of his patents. It was an important event
Since the electric current through the coil in an electriin my professional life. [8]
cal oscillating circuit constantly changes direction, the
Nevertheless, because of Cauer's mathematically chalcapacitor is continually charged or discharged. Depending
lenging procedure, which was highly unusual for electrical
on the choice of capacitors and coils, it is possible to creengineers in the United States at the time, his work was
ate low-pass, high-pass, and bandpass filters that accordlargely ignored. Only a few contemporary network theoingly allow the passage of waves with frequencies below a
reticians made any attempt to look into Cauer's treatment
threshold frequency f1 or above a threshold frequency f2
of electrical filters [7]. In the United States, an ambitious
or in a range between two threshold frequencies f3 and
young electrical engineer, Guillemin (1898-1970), praising
f4 . Filter attenuation bands, which interrupt all frequenCauer, said:
cies outside a particular frequency band between f5 and
...the form of Cauer's publication...is quite mathematif6, are also possible. Electrical filters transmit alternating
cal and, in general, not in accord with the manner in
currents and alternating voltages at frequencies within a
which similar material is presented in this country. [10]
particular frequency band. Oscillations within this range
Guillemin was the first to write a book bearing the title
are allowed to pass, while oscillations outside this passCommunication Networks. The first volume of this book
band are blocked. This is also referred to as the blocking
was published in 1931, and, in it, Guillemin became one
zone of the filter.
of the first in the United States to stress the necessity of
Between 1924 and 1931, research on electrical filters
mathematical abstractions for future research in EE:
was continued primarily at Bell Telephone Laboratories
40

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