Small Direct-Current Electric Motors
Thomas B. Greenslade, Jr.
Kenyon College, Gambier, Ohio 43022
When I was about ten years old I was given an electric motor kit for Christmas. I had to wind a length of insulated copper wire on the rotor, put together the commutator and install a small U-magnet. I enjoyed the construction experience and with some parental assistance figured out how it worked. Ever since then I have had a curiosity about these simple artifacts of technology, and this article discusses some of them that I have met in my scientific travels.
My toy motor was a direct descendent of the motors devised in 1837 by Charles Grafton Page (1812-1868) and built and sold by Daniel Davis, Jr. (1813-1887). The motor in Fig. 1, which was on display at the University of Cincinnati physics department when I photographed it in 2001, is typical of this original design. This must have been widely accepted by the physics teaching community, for it was available in essentially unchanged form until the early years of the 20th century. Figure 2 shows an example of a Page-type motor in the 1916 Knott catalogue.1 It could be laid flat on a sheet of paper and iron filings sprinkled on the traced out the magnetic lines of force from the poles and through the energized armature.
The key elements of Page’s design can be seen in the instruments in Figs. 1 and 2. The rotating armature, wound like a solenoid on a soft-iron core, is pivoted in the middle. The current through the armature is reversed every half turn by the pole-changer mounted on the spinning shaft just above the armature. Today we would use the word commutator instead of Page’s more evocative phrase, “pole changer.” In order to see how rapidly the motor shaft was rotating a bell mechanism (Fig. 3) was used. The clapper stuck the bell every 100 turns. Of course, this slowed down the rotation, defeating the original purpose of the mechanism.
The St. Louis Motor
The St. Louis Motor in Fig. 4 is a descendent of the Page-type motor, with a different geometry that allows it to be used for both lecture demonstrations and laboratory exercises. It was first described in 1909 in an article by S.A. Douglass of Soldan High School in St. Louis, Missouri. Douglass noted that the design was “the outgrowth of a long series of experiments conducted by the physics teachers of the St. Louis High Schools.2
The motor shown in Fig. 4 has a magnetic field produced by a pair of permanent bar magnets. The rotating armature is formed by a half-dozen layers of insulated wire and spins between the north and south magnetic poles. A pair of springy metal brushes contacts a split-ring commutator connected to the end of the armature shaft. The typical operating voltage was about 6 V, often supplied by the tall, circular No. 6 dry cells used for automobile ignition or the more modern Burgess Big Six rectangular battery, specially designed for use in powering battery motors.3
A series of experimental investigations can be built around the St. Louis motor. With the bar magnets in place the effect of changing the magnetic field can be observed by swinging the poles in and out. Reversing the magnetic field by switching the poles of the bar magnets causes the motor to run in the opposite direction, as will reversing the connections to the battery. Changing the current supplied to the motor causes the rotation rate to change.4 The operation of the commutator can be studied by moving the bar holding the brushes back and forth to apply the magnetic torque early or late. Most of these hands-on investigations are in the original article.
The magnetic field can also be supplied by an electromagnet (Fig. 5). A separate battery can be used to energize this coil, but a single battery can be used in two different ways: the armature and the electromagnet can be connected either in series or in parallel. The original article suggests that investigating these two connections is an interesting exercise for the student.
Other Demonstration Motors
The Experimental Motor in Fig. 6 appears in the 1941 Welch catalogue at $6.00. Here, like the St. Louis motor, we get away from the Page-type small motor to one with adjustable parameters. Mounted on the eight-inch diameter cast-iron ring are two horizontal Alnico magnets that may be moved in and out to vary their effect. The permanent magnets may be replaced by electromagnets so that the students can observe the effects of series and parallel-connections. Slip rings for both direct and alternating current commutations are mounted on the axle. There is an interesting note in the catalogue: “if the apparatus is tipped on its back and a sheet of thin white cardboard is slipped between the frame and the place of the magnets, iron filings can be used to show the forms assumed by the magnetic lines of force.”
The Motor and Generation Demonstration Apparatus in Fig. 7 was one I photographed during a visit to the University of Northern Iowa in Cedar Falls, Iowa. From the 1940 Central Scientific Company catalogue: “…a large classroom working model for studying the principle and operation of an electric motor or generator. Operates nicely as either a series or shunt-wound motor. The size and simplicity of the outfit enable an entire class to follow the instructor in tracing the wiring system, and in showing the field magnets, armature, brushes and terminals. Mounted in a rectangular steel frame about 15 inches high and 19 inches wide, attractively finished in gloss-black enamel, with coils colored red and green for indicating polarity … $21.00.”
The “Little-Hustler” motor in Fig. 8 has a long history. The earliest reference I have found is the 1909 Cenco catalogue, where it is listed at $1.40.
For thirty cents more you could get a kit that can be used to build the motor (Fig. 9). In the 1929 catalogue of the Chicago Apparatus Company this has been renamed the “Ajax” motor. The armature has three poles, which means that there are no “dead spots”; the motor will start with the armature in any position, unlike motors with two pole armatures, which lock up when the armature is horizontal. The motors were made by Knapp of New York City, a firm that specialized in toys and games, and was founded in 1895. Most physics apparatus makers in the first half of the 20th century sold this motor, including Welch, Chicago Apparatus Company and L.E. Knott. It could run on a single dry cell or alternating current. And, when fitted with a 5 inch diameter, three-bladed fan, it could be used as a desk fan.
Rotator for Newton’s Color Disks
Sometimes the little D.C. electric motor is hidden, in this case under a rotating color disk. The rotating color top was developed by the English physicist, James Clerk Maxwell (1831-1879) to aid in his studies of color vision and color mixing.5 The motor and four disks in Fig. 10 are listed in the 1929 catalogue of the Chicago Apparatus Company at $1.75. The motor runs nicely on a single dry cell. The commutator consists of two single strands of wire attached lengthwise to the shaft, which means that current is delivered to the armature only during a small fraction of the time. The catalogue copy refers to this indirectly when it notes that when the disk is at rest, there is no drain on the battery. With a small pulley attached, the motor can be used to run small mechanical toys. The motor design was patented on April 9, 1901.
I photographed the direct-current motor in Fig. 11 at Westminster College in western Pennsylvania. There are two curious aspects about it: why is it mounted on a tall base, and why are there four connection terminals, with one suitable for high voltages? This has the label of A.P. Gage of Boston,6 and a look at the 1892 Gage catalogue identified it: this is a rotator for a Geissler tube, and cost $8.00. The magnetic field is supplied by the upright windings, and the armature is of the three-pole type so that the motor will start to run from any angular position. The Geissler tube is clamped to a wooden crosspiece attached to the shaft of the motor, and there is a pair of sliding contacts to apply the high voltage to excite the tube.
Figures 12 and 13 show a rather curious form of electric motor for rotating Geissler tubes. The ring inside which the coils rotate is presumably made of soft iron, and the commutator is arranged so that there is an increasing attraction to it by the energized coils as the gap decreases. The motor in Fig. 12, in the apparatus collection at Union College in Schenectady, New York, has a two pole armature, while the Amherst College example in Fig. 13 has four poles.
The 1896 catalogue of the Chicago Laboratory Supply and Scale Company lists the Amherst motor as a “rotator for Geissler tubes”, and prices it at $9.00 for use with tubes up to 9 inches in length and $24.00 for tubes up to 12 inches in length. It is marked “James W. Queen”, but I have not found it in the 1867 to 1888 catalogues to which I have access. The 1886 catalogue of Curt W. Meyer of New York lists the same apparatus at $6.00, $12.00 and $24.00, depending on the size; the cut is exactly the same as the Chicago Laboratory catalogue.
The Garland Collection of Classic Physics Apparatus at Vanderbilt University has a motor identical to the one at Amherst, and in Robert T. Lagemann’s catalogue of the Garland Collection of Classic Physics Apparatus at Vanderbilt University;7 it is described as “Wheatstone’s motor”. However, I can find no reference to this in biographical material about Charles Wheatstone.
The curious electric motor in Fig. 14 consists of a single layer coil wound on a flat strap of steel, with the upright strips of steel riveted to its ends forming the poles of an electromagnet. Rotating between the poles of the magnet is a piece of iron, which is attracted to the poles when the commutator connects the winding to the power supply. There is enough rotational angular momentum to keep the bar rotating until the next time that electromagnet is energized. This is an example of a Froment-type motor, about which I would like to write more in the future in this journal.
Note that the connections to the ends of the coil are through Fahnestock Clips. This form of spring-clip was developed by John Schaade, Jr., and the February 26, 1907 patent was assigned to the Fahnestock Electric Company.
Direct-current electric motors and generators are essentially the same device; they both have coils of wire rotating in magnetic fields. The 1941 Welch catalogue shows two Little Hustler motors side by side on a wooden base and driven by a belt. However, there was a form of Little Hustler that was specially wound as a generator (Fig. 15). In the figure this is on the left-hand side, although a cursory inspection shows little difference from the motor on the right-hand side. This example, in the collection of Richard Zitto, is listed at $6.00 in the 1936 Chicago Apparatus Company catalogue. There are connections on the base for the input and output voltages; it would be an interesting study to put a load on the output and measure the input power and the power delivered to the load.
The Cambosco Genamotor in Fig. 16 is, as its name suggests, a hybrid device that can be used to demonstrate the basic principles of the motor and the generator. D.C. and A.C. slip-rings are provided for use with both types of power supply. The advertisement in Fig.17 describes its use, and also shows the permanent magnet that also can be used to produce the magnetic field.
The mechanism of a D’Arsonval-type electric meter has some features in common with the direct-current electric motor: a current carrying loop of wire moves in the field of a permanent magnet. This is also the mechanism of the D.C. generator. In the mid-1970s two Kenyon colleagues, Franklin Miller and John Johnson, used this idea for a demonstration8 in which two small D.C. zero-center meters were connected together. Picking up one and twisting it caused the needle mechanism to turn, moving the coil in the magnetic field. This was just the same as Michael Faraday’s 1831 demonstration of induced voltages, and the resulting current caused the second meter to indicate its presence.
- This Page-type motor is in the 1916 catalogue of the L.E. Knott Apparatus Co. of Boston on pg 395. ↩
- S.A. Douglass, “The St. Louis Laboratory Motor”, School Science and Mathematics, 9 (1909), 678-681. ↩
- Thomas B. Greenslade, Jr., “The St. Louis Motor”, Phys. Teach., 49 (2011) 424-425. ↩
- Thomas B. Greenslade, Jr., “Devices to Demonstrate Electromagnetic Rotation”, Phys. Teach., 34 (1996), 412-416. ↩
- Allan Mills, “Colour Matching and Mixing With Particular Reference to Maxwell’s Disc”, eRittenhouse, 27 (2016). ↩
- Thomas B. Greenslade, Jr., “The Apparatus of Alfred P. Gage”, eRittenhouse, 26 (2016) ↩
- Robert T. Lagemann, The Garland Collection of Classical Physics Apparatus at Vanderbilt University (Folio Publishers, Nashville, Tennessee, 1983), pg 142. ↩
- Franklin Miller, Jr. and John A. Johnson, “A motor is a generator and vice versa”, Phys. Teach., 14 (1976), 133. ↩