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Electricity Makes Things Move
From Faraday's electromagnetic rotation to the practical DC motor
The telegraph used an electromagnet to make a small mechanical motion at the end of a wire. But electricity could do much more than click a sounder or pull an armature. It could produce continuous rotation.
Faraday's electromagnetic rotation — 1821
This part of our story actually begins before the mature telegraph systems of the 1830s and 1840s. Soon after Ørsted and Ampère revealed the relationship between current and magnetism, Michael Faraday asked what would happen if the magnetic force were allowed to produce unrestricted movement.
In 1821 Faraday demonstrated continuous electromagnetic rotation. In one arrangement, a current-carrying wire moved in a circle around a magnet. Electrical energy had been converted into continuous mechanical motion.
electrical energy → magnetic interaction → mechanical motion
From a laboratory curiosity to a rotating machine
Faraday's apparatus proved the principle, but it was not yet a useful shop motor. Practical rotating machines required stronger magnets, coils arranged to produce torque, bearings, a rotating armature, and a way to keep the torque acting in the proper direction as the rotor turned.
The commutator solves a directional problem
Suppose a current-carrying coil is placed in a magnetic field. The coil experiences a torque, but after it turns about half a revolution the same current direction would tend to drive it back the other way.
A commutator is a rotating electrical switch. Together with stationary brushes, it reverses the current in the rotating coil at the proper point in each revolution so that the torque continues in the same rotational direction.
Thomas Davenport
In the 1830s the American blacksmith and inventor Thomas Davenport built practical electromagnetic rotating machines. His 1837 U.S. patent was the first U.S. patent issued for an electric motor.
Davenport's machines already contained recognizable motor ideas: electromagnets, a rotating member, switching of current, and mechanical output. He even demonstrated a small electrically driven model railway.
Others were pursuing the same problem
Inventors in several countries worked on electromagnetic motors during the same period. Moritz von Jacobi, for example, developed powerful motors and demonstrated an electric boat in the 1830s.
But batteries were expensive sources of energy. Steam engines remained much more practical for heavy industrial power. The electric motor would become far more important after generators made electrical energy cheaper and more abundant.
Measurement trail 12 — power: the watt (W)
A motor makes us ask a new question. Voltage and current tell us about the electrical conditions, but how fast is energy being delivered?
That quantity is power. The SI unit is the watt, symbol W, named for steam-engine pioneer James Watt.
Unlike the ampere or volt, the watt does not need its own independent physical artifact. Once the joule, second, volt, and ampere are defined, the watt follows from their relationships. The watt became part of the internationally standardized electrical unit system in the late nineteenth century.
A simple motor-input example
If a motor operates from 12 V and draws 2 A:
That is 24 joules of electrical energy delivered each second. A real motor converts only part of that input into useful shaft power; the rest becomes heat, sound, magnetic loss, friction, and other losses.
James Watt had earlier popularized horsepower as a mechanical comparison. One mechanical horsepower is approximately:
This gives us a direct bridge between the older mechanical-power world and electrical machines.
Motor action and generator action belong together
A conductor carrying current in a magnetic field can experience force. A conductor moving through a magnetic field can have an EMF induced in it. Those are not unrelated tricks — they are two sides of electromagnetic energy conversion.
| Motor | Generator |
|---|---|
| Electrical input | Mechanical input |
| Magnetic force produces torque | Motion through magnetic field induces EMF |
| Mechanical output | Electrical output |