Walter Baily's Motor

Lore & Background

Before Baily's invention, Arago's rotations of a copper disk were produced only by rotating a steel magnet beneath it. Baily instead used a fixed electromagnet with four poles, causing its magnetism to shift progressively between them. This induced eddy currents in a pivoted copper disk, making it rotate in the direction of the pole progression. The model's disk was about 2+3/8 inches in diameter, with four magnet cores 4 inches high joined to a common yoke, each wound with about 150 turns of 2.5 mm insulated copper wire. The coils were connected in two series pairs, like two independent horseshoe magnets set diagonally. A revolving commutator with springs and contact strips on a wooden base, turned by a wire handle, reversed currents from two batteries alternately in the two circuits, producing successive polarities in the four poles.

Baily clearly articulated that the disk's rotation was due to the rotation of the magnetic field. He noted that if a similar motion of the field could be produced by other means, the disk would similarly rotate. He discussed that an intermittent rotation of the field via electromagnets could drive the disk, and suggested that a circle of poles successively excited in opposite pairs would give impulses all in one direction. He pointed out that with an infinite number of electromagnets, a uniform rotation of the magnetic field would result, as with rotating permanent magnets. In his actual model with two pairs of poles, reversing the order of excitation changed the rotation direction, achievable by reversing the commutator action or one battery's connections. The original paper diagram suggested laminated iron cores, but the actual model used solid cores. Baily remarked that placing four more electromagnets above the disk, each opposite a lower one with opposing polarity, could increase the effect. The model ran well with four dry cells. When Prof. Guthrie jokingly asked about the motor's power, Baily modestly replied he could only regard it as a scientific toy.

Reader's Guide

Walter Baily's motor holds significance as a foundational demonstration of the rotating magnetic field principle, which is central to polyphase and induction motors. The article notes that Baily's 1879 exhibition marks the beginning of modern polyphase motor development. By using fixed electromagnets and a commutator to create a shifting magnetic field, Baily showed that a copper disk could be rotated without moving magnets, directly illustrating the concept of a rotating field. His clear explanation that the disk's motion was due to the field's rotation, and that an infinite number of poles would yield uniform rotation, anticipated later polyphase systems. The motor's design—with four poles, two circuits, and a commutator—was a primitive but effective proof of concept. Baily's modest characterization of it as a 'scientific toy' reflects its early stage, yet the article positions it as a key step in the timeline of electric motors. The legacy, as described, is that it provided a practical method to produce rotation via electromagnets, influencing subsequent developments in alternating-current motors and polyphase power systems.

Did You Know?

• Baily's motor used fixed electromagnets instead of a rotating steel magnet to produce Arago's rotations.
• The model's commutator was built from springs and contact strips mounted on a bit of wood, turned by a wire handle.
• Baily suggested that placing four additional electromagnets above the disk could increase the effect.