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2011
The St. Louis Motor
Tom Greenslade
Kenyon College, [email protected]
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Recommended Citation
“The St. Louis Motor”, The Physics Teacher, 49, 424-425 (2011)
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The St. Louis Motor
Thomas B. Greenslade Jr.
Citation: The Physics Teacher 49, 424 (2011); doi: 10.1119/1.3639150
View online: http://dx.doi.org/10.1119/1.3639150
View Table of Contents: http://scitation.aip.org/content/aapt/journal/tpt/49/7?ver=pdfcov
Published by the American Association of Physics Teachers
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The St. Louis Motor
Thomas B. Greenslade Jr., Kenyon College, Gambier, OH
T
he St. Louis Motor, invented in 1909, is unique among
physics apparatus for being named for a geographical
place rather than a physicist. The sturdy little device
(Fig. 1) has never been out of production. Any older school
or physics department that has not done a catastrophic
housecleaning in the last 20 years will certainly have a small
flock of them in the back room.
The basic motor in Fig. 1 has a magnetic field supplied by
a pair of permanent magnets. The rotating armature is a coil
with a half-dozen layers of insulated wire that spins between
the north and south poles of the magnets. A split-ring commutator attached to the ends of the armature coil is contacted
by springy metal brushes that are connected to the positive
and negative poles of a power supply.
The original 1909 article describing the motor in School
Science and Mathematics is by S. A. Douglass of Soldan High
School in St. Louis.1 The school opened in that year and it is
still in operation as a magnet school specializing in international studies. The motor was “the outgrowth of a long series
of experiments conducted by the physics teachers of the St.
Louis High Schools.” Martin Taylor, the former president
of the Central Scientific Company, suggested that work on
the motor started as early as 1907 and that it was first made
by Cenco. That seems reasonable, as the cut of the motor in
Douglass’ article has “Central Scientific Co.” written across the
base of the motor, and the 1909 Cenco catalogue uses the same
Fig. 1. The basic St. Louis Motor. This example sold
for $4 in the 1929 Chicago Apparatus Co. catalog
and is in the Greenslade Collection.
424
figures as the School Science article.
What experiments could you do with this device? The
first thing to note is that it is also a generator. If you connect a
sensitive galvanometer to the terminals and spin the rotor, the
galvanometer will deflect either right or left, depending on the
direction of spin. Indeed, it is entertaining to connect two of
the motors together—when you rotate one of them manually,
the armature of the other will start to rotate.
Douglass’ article lists several experiments that can be done
with the motor. Reversing the magnetic field by switching the
magnets around will cause the motor to run in the opposite
direction, as will reversing the current connections to the
brushes. The two bar magnets are held in rotating clips, and
so the strength of the magnetic field and hence the magnetic
torque on the armature’s electromagnet can be varied. The
rate of rotation can also be varied by changing the current
supplied to the motor.
The motor usually came with an electromagnet to supply
the magnetic field (Fig. 2). The original article notes that “by
making the proper connections, the machine may be made
either parallel wound [the magnet coil in parallel with the
armature] or series wound [the magnet coil in series with the
armature] motor. An interesting fact that may be brought out
is that if the motor is operated first with the parallel arrangement … and then be changed to the series arrangement …
leaving the positive wire connected to the same binding post
Fig. 2. The magnetic field could be supplied by an
electromagnet. This example is by C.H. Stoelting
of Chicago and sold for $2 in 1912. It is in the
Greenslade Collection.
The Physics Teacher ◆ Vol. 49, October 2011
DOI: 10.1119/1.3639150
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as before, the rotation of the armature will be reversed. This
provides a simple little electrical problem for the student to
explain.”
Quite soon apparatus manufacturers were supplying St.
Louis Motors with two armatures (Fig. 3). The one for direct
current on the left has the split-ring commutator, while the
alternating current armature on the right has slip rings. The
1933 Welch catalogue lists a four-pole armature in place of the
standard two-pole model to show that the motor starts more
easily with extra poles.
The St. Louis motor has an illustrious ancestor—the rotating armature device in Fig. 4, devised in 1837 by the American
electrical inventor Charles Grafton Page (1812-1868) and first
made by the Boston apparatus manufacturer Daniel Davis Jr.2
The figure is drawn from the 1848 edition of Davis’ Manual of
Magnetism. Here you can see the permanent magnet, the rotating armature, and the commutator (called the “pole changer”
by Page, who invented it) that make up the essential parts of
this first rotating electrical motor. The core of the armature
was originally made of wood and later changed to soft iron.
For the rest of the 19th century, these basic Page-type motors were used to drive siren disks, rotating mirrors, Newton’s
color disks, rotating Geissler tubes, etc.
In working through my sources, I found a long series of
experiments with the St. Louis Motor in a 1914 text by Cyril
M. Jansky.3 The name was familiar, and I soon found that
Jansky’s son was Karl Guthe Jansky (1905-1950), often called
the founder of radio astronomy. Karl Guthe was Cyril Jansky’s
teacher and colleague (and textbook author) at the University
of Wisconsin.
The St. Louis Motor is still in production. Be sure to have at
least two in your apparatus collection for demonstrations and
experiments.
References
1. S. A. Douglass, “The St. Louis laboratory motor,” School Sci.
Math. 9, 678–681 (1909).
2. Thomas B. Greenslade Jr., “Devices to display electromagnetic
rotation,” Phys. Teach. 34, 412–416 (Oct. 1996).
3. Cyril M. Jansky, Elementary Magnetism and Electricity (McGraw-Hill, New York, 1914).
Thomas B. Greenslade Jr. is professor emeritus in the physics department at Kenyon and a frequent author for The Physics Teacher.
Physics Department, Kenyon College, Gambier, OH 43022;
[email protected]
Fig. 3. Rotating armatures designed for dc operation (left)
and ac operation (right). These were made by Stoelting,
and the ac armature cost $1 in 1912. They are in the
Greenslade Collection.
Fig. 4. Page’s Revolving Electromagnet,
from Daniel Davis Jr., Manual of
Magnetism (Boston, 1848), p. 212.
The Physics Teacher ◆ Vol. 49, October 2011
425
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