IEEE Power & Energy Magazine - January/February 2021 - 89

from a superficial examination
of the design and construction,
and it is with the idea of throwing some light on this seeming
mystery that the investigation to
be described was undertaken....
Is the machine well suited for arc
lighting? The vast numbers of
these dynamos in daily use in all
parts of the world is a practical
answer which must carry more
weight than any which might be
suggested by a theoretical study
of the machine.... It is to be regretted that time did not permit
the measurement of the exact
electromotive force in a separate
coil of the armature ...

Brush Arc Lamps
Brush filed his first patent application for an arc lamp on 28 September
1877; patent 203,411 was issued on
7 May 1878. Brush arc lamps were
very successful because of their simple construction, which included no
clockwork (Figure 6). Up to 16 or more
Brush arc lamps could be operated in
series, powered by the Brush No. 7
dynamo, with each carbon set lasting
roughly 8 h. Lamps with two or three
carbon sets for unattended 16- or 24-h
operation were available, in which a
Brush-invented mechanism automatically switched from one carbon set
to another. It is noteworthy that the
Brush No. 7 dynamo and larger ones
had to be high-voltage generators to
power arc lamps in series. The voltage
drop across a Brush arc lamp was typically about 50 V, so a 16-lamp system
would require an 800-V dynamo. The
40-light No. 8 dynamo (Figure 4) operated at 2,000 V.
Brush also developed carbons that
were more suitable for the large-scale
use of arc lighting than existing ones.
Thin carbon points provide better illumination than thick ones, but they were
not practical before Brush's invention
because of their high resistance and
rapid consumption. Brush solved the
problem by copper-coating the carbons
(U.S. patent 196,425, filed on 21 August 1877 and issued on 23 October
january/february 2021	

figure 5. The wiring diagram of the Brush No. 7 dynamo. (Adapted from Silvanus
P. Thompson, Dynamo-Electric Machinery, 2nd ed. London: E. & F.N. Spon, 1886.)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

figure 6. (a) The Victor Serrin arc lamp, circa 1860. (Source: A. Guillemin,
The Application of Physical Forces. London: Macmillan, 1877.) Complex arc
regulators, such as Serrin's, became obsolete when Brush arc lamps entered
the market. Brush arc lamps and accessories in 1880: (b) an arc lamp with two
carbon sets, (c) a package of 25 carbons, (d) a lamp with a single carbon set, (e)
a focusing lamp for projections by magic lanterns, (f) a headlight lamp for use
in reflectors on ships and locomotives, (g) a dial attachment for controlling how
many lamps in series are burning, and (h) an ornamental lamp. (Source: The
Brush Electric Light. Cleveland, Ohio: Brush Electric Company, 1881.)
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IEEE Power & Energy Magazine - January/February 2021

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