Perhaps the greatest theoretical achievement of physics in the 19th century was the discovery of electromagnetic waves. The first hint was an unexpected connection between electric phenomena and the velocity of light.
Electric forces in nature come in two kinds. First, there is the electric attraction or repulsion between (+) and (-) electric charges. It is possible to use this to define a unit of electric charge, as the charge which repels a similar charge at a distance of, say, 1 meter, with a force of unit strength (actual formulas make this precise).
But second, there is also the attraction and repulsion between parallel electric currents. One could then define the unit of current, as the current which, when flowing in a straight wire, attracts a similar current in a parallel wire 1 meter away with a force of unit strength, for every meter of the wires’ length.
But electric current and charge are related! We could have just as well based the unit of current on the unit of charge—say, as the current in which one unit of charge passes each second through any cross section of the wire. This second definition turns out to be quite different, and if meters and seconds are used in all definitions, the ratio of the two units of current turns out to be the speed of light, 300,000,000 meters per second.
In Faraday’s time the speed of light was known, although not as accurately as it is today. It was first derived around 1676 by Ole (Olaus) Roemer, a Danish astronomer working in Paris. Roemer tried to predict eclipses of Jupiter’s moon Io (mentioned later here in an altogether different connection) and he found a difference between actual and predicted eclipse times, which grew and then decreased again as the Earth circled the Sun. He correctly guessed the reason, namely, as the Earth moved in its orbit, its distance to Jupiter also went up and down, and light needed extra time to cover the extra distance...Read on...http://www.sciencemaster.com/jump/physical/magnetic_waves.php
Posted by Jay Roberts at 05:28 AM | Permalink