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12.   The Dynamo Process

A dynamo (or electric generator) is a device converting
the energy of motion to that of an electric current.

Index

4. A Gilbert Expt.

5. Before Gilbert

6. London in 1600

7. 1600-1820

8. Oersted & Ampére

9. The Lodestone

10. Gauss

11. The Magnetic Sun

12. Fluid Dynamos

13. Dynamo in the
Earth's Core

14. Magnetometers and
Tobacco Smoking

15. Magnetic Reversals
& Moving Continents

16. The Magnetosphere

17. Magnetic Planets
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 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").

So Many Dynamos...   *

 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".)
 * now read it backwards Exploring Further     The book "Michael Faraday and the Nature of Electricity" by Roberta Baxter (144 pp, Morgan Reynolds publishing, Greensboro, North Carolina, 2009) tells of Faraday's life and work. A slim book probably meant for teen-agers, it is nevertheless rather detailed and quite readable. Optional     (Mainly for Teachers)     When the shaft of a dynamo is turned by some outside force, its electrical connections can produce an electric current. Many dynamos however are reversible: if an electric current is fed into its electrical connection, the force on the current can turn the same shaft, converting the dynamo into a motor. In some models of "hybrid" automobiles, electric motors fed by a storage battery help the gasoline engine accelerate the car, but then when the car slows down, they become dynamos and feed an electric current back into the battery, saving energy which otherwise might turn into waste heat in brakes.     The Faraday disk dynamo can also reverse and become a motor, though a rather inefficient one. A tabletop demonstration of this is described in "Magnetic Force on an Electrolyte" by Marián Kireš and Zuzana Ješkova, on p. 50-51 of "The Physics Teacher," Vol. 45, January 2007.     The rotating disk here is a shallow layer of water containing copper sulfate, which conducts electricity. The solution in put in a Petri dish and placed on top of one of the poles of a vertical laboratory electromagnet; little pieces of cork floating on the fluid then trace its rotation     The electrodes are two circles of copper (they can be sawed from the ends of pipes)--a big one on the outer boundary of the solution and a small one in the middle. When the terminals of a battery of an electric power supply are connected to the two copper circles, the fluid starts rotating. Reverse the contacts and the rotation reverses too. Questions from Users:   "Why does this happen?" (electromagnetic induction)                       ***         Can we tell if a symmetric magnetic field rotates around its axis?                   ***         Dynamo theory               ***         Earth--conductor or insulator?           ***         Rapidly reversing magnet       ***         How can an intensely hot Sun be magnetic? ***         Electric field due to electromagnetic induction
Author and Curator:   Dr. David P. Stern
Mail to Dr.Stern:   earthmag("at" symbol)phy6.org
Last updated 12 April 2007