12.   The Dynamo Process

The concept goes back to Michael Faraday (1791-1867). The son of a poor blacksmith, Faraday was apprenticed to a bookbinder and taught himself by reading books brought to be rebound. He rose to become Britain's foremost scientist, famous for his brilliant insights and his popular lectures. Faraday discovered that by moving a magnet next to a closed electric circuit, or changing the magnetic field passing through it, an electric current could be "induced" to flow in it. That "electromagnetic induction" remains the principle behind electric generators, transformers and many other devices.

Faraday's disk dynamo

Faraday showed that another way of inducing the current was to move the electric conductor while the magnetic source stood still. This was the principle behind his disk dynamo, which featured a conducting disk spinning in a magnetic field--you may imagine it to be spun up by some belt and pulley, not drawn here. The electric circuit was then completed by stationary wires touching the disk on its rim and on its axle, shown on the right side of the drawing. This is not a very practical dynamo design (unless one seeks to generate huge currents at very low voltages), but in the large-scale universe, most currents are apparently produced by motions of this sort.

The Waterloo Bridge experiment

Faraday's moving conductor of electricity was solid (e.g. a copper disk), but a circulating fluid can also create such currents. Faraday was aware of the possibility of such "fluid dynamos," and accordingly he tried to measure the electric current created by the flow of London's river Thames across the Earth's magnetic field. He stretched a wire across London's Waterloo bridge (drawing), dipped its ends into the river and tried to measure the induced flow of electricity (curved line of small arrows). Small voltages due to chemical processes prevented him from observing the effect, but the idea was sound.

Faraday even speculated (incorrectly) that the flow of the Gulf Stream in the Atlantic Ocean was somehow electrically linked to the high atmosphere, powering there an electric discharge which (in his view) was the polar aurora ("Northern Lights").

For some unusual examples of this dynamo process, see the web sites about the dynamo involving Jupiter's moon Io and about the space tether experiment on the Space Shuttle.

Faraday's disk dynamo needs a magnetic field in order to produce an electric current. Is it possible for the current which it generated to also produce the magnetic field which the dynamo process required? That, in a nutshell, was what Larmor proposed was happening in sunspots.

At first sight this looks like a "chicken and egg" propostion: to produce a chicken, you need an egg, but to produce an egg you need a chicken--so which of these came first? Similarly here--to produce a current, you need a magnetic field, but to produce a magnetic field you need a current. Where does one begin? Actually, weak magnetic fields are always present and would be gradually amplified by the process, so this poses no obstacle.

One could, for instance, link two Faraday dynamos, each supplying the current needed to produce the other's magnetic field. They could (in principle) form a feasible self-excited dynamo, deriving its energy from whatever force was turning the disks. However, whether a fluid dynamo could mimic this behavior--e.g. swirls of fluid flow in a tank of liquid mercury--is a much harder question and took decades to resolve.

(More about dynamos in the next section and in part #14 of
"A Millennium of Geomagnetism".)

Next Stop:   13. The Self-Sustaining Dynamo in the Earth's Core