Rub a balloon on your hair and it will cling to a wall. Hold a magnet near a compass and the needle swings. For most of history these looked like two unrelated curiosities. The great discovery of the nineteenth century was that they are two faces of one force, and that discovery built the modern world of lights, motors, radio and power grids.

Electric charge

The ancient Greeks noticed that amber rubbed with fur attracts bits of straw; our word electricity comes from elektron, the Greek word for amber. Matter contains two kinds of electric charge, which Benjamin Franklin named positive and negative in the 1740s. Like charges repel each other and opposite charges attract. Ordinary atoms hold equal amounts of both: positive protons in the nucleus and negative electrons around it. Rubbing a balloon on your hair moves some electrons from the hair to the balloon, leaving both charged, which is why your hair then stands up and reaches toward it. Charge is conserved: it can be moved from one object to another, but never created or destroyed.

In 1785 the French engineer Charles-Augustin de Coulomb measured that the electric force weakens with distance in the same way gravity does, falling to a quarter when the distance doubles. Yet it is vastly stronger. The electric push between two protons is roughly a trillion trillion trillion times their gravitational pull. Gravity rules the large scale only because big objects are almost perfectly neutral, so their electric forces cancel out.

Current, voltage and resistance

In 1800 the Italian physicist Alessandro Volta stacked disks of zinc, copper and cloth soaked in salt water and built the first battery, producing a steady flow of charge for the first time. Electric current is the flow of electric charge; in a metal wire it is carried by electrons. Voltage is the push that drives current around a circuit. A helpful picture compares a circuit to water in pipes: voltage is like the water pressure, and current is like the rate of flow. Current flows only when there is a voltage difference between two points and a conducting path between them.

Resistance is how strongly a material opposes the flow of current, turning some electrical energy into heat; that is how a toaster or an old style light bulb works. Conductors such as copper let charge move freely, while insulators such as rubber and glass hold their electrons in place, so charge cannot flow. That is why copper wires wear plastic coats.

The great unification

In 1820 the Danish physicist Hans Christian Ørsted noticed, during a lecture, that a wire carrying current made a nearby compass needle swing. Ørsted's discovery showed that an electric current produces a magnetic field. Within months André-Marie Ampère in Paris had worked out the laws of the effect. It is how an electromagnet works, and how an electric motor turns electricity into motion.

If electricity could make magnetism, could magnetism make electricity? Michael Faraday, a former bookbinder's apprentice with little formal schooling who became the greatest experimenter of his age, found the answer in 1831. A magnet sitting still beside a coil of wire did nothing. But a changing magnetic field, such as a magnet moving in and out of a coil, produces an electric current, an effect called electromagnetic induction. Nearly every power station on Earth relies on it. Coal, gas, nuclear, wind and hydroelectric plants all spin magnets and coils past each other in a generator; they differ only in what turns the shaft. Alternating current soon carried this power across cities.

Between 1861 and 1865 the Scottish physicist James Clerk Maxwell gathered these findings into a short set of equations. They predicted that changing electric and magnetic fields could sustain each other and travel through space as a wave. When Maxwell worked out the speed of that wave, it matched the measured speed of light. He concluded that light itself is an electromagnetic wave. In the late 1880s Heinrich Hertz in Germany produced and detected such waves in his laboratory, the first radio waves made on purpose. Three subjects, electricity, magnetism and light, had become one.

Worked example: why the bird on the wire is safe

A sparrow perches on a bare power line carrying thousands of volts, and nothing happens. Why? Both of its feet touch the same wire at nearly the same voltage, so there is almost no voltage difference across its body and almost no current flows through it. The danger comes when something bridges the line and the ground, or two lines at different voltages. A large bird whose wings touch two wires at once, or a person on a metal ladder who touches a line while standing on the earth, becomes a path for current, and the result can be fatal.

The same idea explains household safety rules. A ground wire gives stray current an easy path into the earth, so that it does not pass through you. A circuit breaker cuts the power when far too much current is flowing. Electricity is not dangerous because of voltage alone, but because a voltage difference can drive current through a body.