Barlow's wheel

Lore & Background

Barlow's wheel was designed and built by English mathematician and physicist Peter Barlow in 1822. The apparatus consists of a star-shaped copper wheel capable of rotating freely in a vertical plane about a horizontal axis. The point of each spoke of the star just dips into a pool of mercury kept in a small groove on the wooden base of the apparatus, between the two opposite poles of a strong magnet. The wheel rotates with its plane perpendicular to the direction of the magnetic field, and during its rotation only one point of the star dips into the pool of mercury at a time.

When the axis of the wheel and the mercury are connected to an electric cell, the circuit is completed through the axis of the wheel and the mercury. On passing current, the wheel begins to rotate due to the action of the magnet on the current. The direction of rotation can be determined by applying Fleming's left-hand rule. While rotating, when a spoke leaves the mercury the circuit breaks, but due to inertia of motion the wheel continues and brings the next spoke into contact, restoring electrical contact. The speed of rotation depends upon the strength of the magnetic field and the strength of the current.

Reader's Guide

Barlow's wheel is used as a demonstration of electromagnetism in physics education. The article notes that a more rapid revolution will be obtained if a small electro-magnet be substituted for a steel magnet, fixed to the stand and included in the circuit with the spur-wheel, so that the current flows through them in succession. In that configuration, the direction of rotation will not be changed by reversing that of the current, since the polarity of the electromagnet will also be reversed. On reversing the direction of the current or that of the magnetic field in the standard setup, the wheel rotates in the opposite direction. Because mercury is toxic, brine is sometimes used in place of mercury in modern recreations of the experiment. The device demonstrates how mechanical energy is obtained from electrical energy.

Did You Know?

• The wheel's points dip into a pool of mercury between the poles of a horseshoe magnet.
• If an electromagnet is substituted for a steel magnet, reversing the current does not change the rotation direction.

Origins and Mechanical Design

Peter Barlow, an English mathematician and physicist, conceived and constructed this device in 1822 as one of the earliest practical demonstrations of what we now call a homopolar motor. The core of the apparatus is a star-shaped copper wheel mounted so it can spin freely in a vertical plane around a horizontal axis. Its pointed spokes extend downward into a shallow groove carved into a wooden base, where a pool of liquid mercury collects. This groove sits squarely between the two poles of a horseshoe magnet, positioning the wheel's plane perpendicular to the magnetic field. When a direct current is connected between the wheel's central hub and a separate electrical contact resting in the mercury, a complete circuit forms through whichever spoke is currently submerged. The Lorentz force exerted by the magnetic field on the moving charges within the copper then drives the wheel into continuous rotation, converting electrical energy into mechanical motion.

The Cycle of Rotation

The wheel's continuous spin relies on a clever interplay between electrical contact and mechanical inertia. At any given instant, only a single spoke tip is submerged in the mercury pool, completing the circuit through the hub, that one spoke, the liquid metal, and the external contact. As the wheel turns and that spoke lifts out of the mercury, the circuit momentarily breaks. However, the wheel's rotational inertia carries it forward just enough for the next spoke to dip into the pool, re-establishing the electrical path and allowing the Lorentz force to act once more. This rhythmic make-and-break of the circuit, sustained by momentum, is what keeps the rotation going without any commutator or switching mechanism. The direction in which the wheel spins can be predicted by applying Fleming's left-hand rule to the geometry of the field and current. Flipping either the current direction or the magnetic field polarity reverses the rotation, while the speed of revolution scales with the strength of both the field and the applied current.

Electromagnet Enhancement and Directional Stability

A notable refinement to the original design involves replacing the static steel horseshoe magnet with a small electromagnet mounted directly on the stand. In this modified arrangement, the electromagnet is wired into the same circuit as the star wheel, so that the driving current passes through both components in sequence. This substitution yields a noticeably faster revolution because the electromagnet produces a stronger field than a permanent steel magnet. More interestingly, the configuration introduces a built-in directional stability: if an operator reverses the direction of the supply current, the polarity of the electromagnet flips simultaneously. Because both the field and the current reverse together, the net Lorentz force on the wheel's charges points the same way, and the wheel continues rotating in its original direction rather than reversing. This self-consistent behavior makes the electromagnet version a particularly elegant teaching tool, illustrating how coupled field and current changes can preserve a fixed sense of rotation.

Classroom Legacy and Safer Alternatives

More than two centuries after Barlow first set his copper star spinning, the wheel remains a staple of introductory physics instruction, serving as a tangible, hands-on demonstration of how a magnetic field exerts force on a current-carrying conductor to produce rotational motion. In lecture halls and laboratories worldwide, students observe the direct conversion of electrical energy into mechanical energy in a single, self-contained apparatus. However, the original design's reliance on liquid mercury presents a genuine safety concern in modern educational settings, given the well-documented toxicity of mercury vapor and its environmental persistence. To address this, many contemporary recreations of the experiment substitute a brine solution for the mercury pool, providing a conductive liquid medium that allows the circuit to close while eliminating the health hazard. This adaptation preserves the essential physics—the Lorentz force, the intermittent circuit, the inertial carry-through—while making the demonstration accessible and safe for repeated classroom use.