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Electricity Makes Magnetism
The magnetic effect of current bends around the wire
A battery made it possible to maintain current through a wire. That simple ability led to a discovery that permanently joined two subjects that had looked separate for centuries: electricity and magnetism.
Ørsted and the compass needle
In 1820 the Danish physicist Hans Christian Ørsted observed that a compass needle moved when electric current flowed through a nearby wire.
The effect was not what one might first expect. The magnetic influence did not simply point from one end of the wire to the other.
A straight current produces circular magnetic-field lines surrounding the conductor.
A field with direction
Reverse the direction of current and the direction of the magnetic field also reverses. A compass placed at different positions around the wire points in different directions because it follows the local magnetic field.
The right-hand rule
- Point the thumb of your right hand in the direction of conventional current.
- Curl your fingers around the wire.
- Your fingers curl in the direction of the magnetic field.
Ampère studies the relationship
The French physicist André-Marie Ampère quickly began investigating Ørsted's discovery. He studied the forces between current-carrying conductors and developed mathematical descriptions of the relationship between electricity and magnetism.
Measurement trail 6 — electric current: the ampere (A)
Electric current is the rate at which charge passes a point. Its unit is the ampere, symbol A, named for Ampère.
So a current of one ampere moves one coulomb of charge past a point each second. This also completes the relation previewed earlier:
The standard itself has changed with measurement technology. The 1893 “international ampere” was realized by electrolysis: the specified current deposited about 0.001118 gram of silver per second from silver nitrate. From 1948 the SI ampere was tied to the magnetic force between parallel wires. Since 2019 it has been defined through the fixed value of the elementary charge e.
| Current arrangement | Observed tendency |
|---|---|
| Parallel currents in the same direction | The conductors attract. |
| Parallel currents in opposite directions | The conductors repel. |
Ohm measures what opposes current
In 1827, between Ørsted's discovery and Faraday's induction work, the German physicist Georg Simon Ohm published his quantitative study of electrical circuits. He showed the relationship among potential difference, current, and what we now call resistance.
Measurement trail 7 — resistance: the ohm (Ω)
Resistance tells us how strongly a circuit element opposes current. Its unit is the ohm, symbol Ω.
A resistance of one ohm allows one ampere of current when one volt is applied across it. The same relationship is the familiar form of Ohm's law:
The nineteenth century needed a physical resistance that laboratories could reproduce. The “international ohm” was represented by the resistance of a specified column of mercury: 106.3 cm long, containing 14.4521 g of mercury, at 0°C, with constant cross-section. Modern resistance standards are vastly more precise and can be realized using quantum effects.
Bend the wire into a loop
If a straight wire produces circular magnetic fields around itself, bending the wire into a loop causes the magnetic effects from different parts of the loop to reinforce one another through the center.
The solenoid
Wind many turns of wire into a long coil and the resulting device is called a solenoid. Its magnetic field resembles that of a bar magnet, with a north and south magnetic direction.
The field can be strengthened by:
- increasing the current;
- adding more turns of wire;
- and placing suitable magnetic material, such as iron, inside the coil.
The electromagnet
A coil around iron can create a strong temporary magnet. Unlike a permanent magnet, an electromagnet can be turned on and off by controlling current.
electricity could now produce mechanical force on command.
Electromagnets would become central to telegraph sounders, relays, bells, solenoids, motors, telephone receivers, loudspeakers, and countless control devices.
The reverse question
The obvious challenge was now:
That question would occupy one of the greatest experimental minds of the nineteenth century: Michael Faraday.