FIG. 24–TWO COILS WITH CURRENTS FLOWING IN SAME DIRECTION ATTRACT EACH OTHER
FIG. 25–TWO COILS WITH CURRENTS FLOWING IN OPPOSITE DIRECTIONS REPEL EACH OTHER
Ampère believed that electric currents are flowing around within the earth, and that the earth has a north and a south magnetic pole for the same reason that a coil of wire has magnetic poles; that these poles are caused by the currents flowing around in the earth just as the poles of the coil are caused by the current flowing around in the coil.
We do honor to the name of Ampère whenever we measure an electric current, for electric currents are measured in "amperes."
Arago
Another important discovery was made by a young Frenchman, François Arago, within a year of the time when Oersted and Ampère made their discoveries. The three great discoveries of these men were made in the years 1819 and 1820. The youth of Arago was full of adventure. He had assisted in making a survey in the Pyrenees, the haunt of daring robber-bands. Twice in his cabin he was visited by a chief of a robber-band who claimed to be a custom-house guard. On the second visit he said to the robber: "Your position is perfectly known to me. I know that you are not a custom-house guard. I have learned that you are the chief of the robbers of the country. Tell me whether I have anything to fear from your confederates." The robber replied: "The idea of robbing you did occur to us; but, on the day that we molested an envoy from the French, they would direct against us several regiments of soldiers, and we are not so strong as they. Allow me to add that the gratitude which I owe you for the night's shelter is your surest guarantee."
At a later time, when war between Spain and France was threatened, he was accused of being a spy, and a mob was formed to put him out of the way. He escaped in disguise through the midst of the mob and boarded a Spanish ship. He was carried to Morocco, ran the gantlet of bloodthirsty Mussulmans in Algiers, escaped death by a hair's-breadth, and through it all clung to the papers which recorded the results of the survey in the mountains, and delivered them in safety to the office of the Bureau of Longitude in Paris.
Arago made a discovery which, with those of Oersted and Ampère, prepared the way for Faraday's great electrical discoveries and the invention of the dynamo. He found that a plate of copper whirling above or below a magnetic needle will draw the needle after it (Fig. 26). He could make the speed of the whirling copper plate so great that the needle would whirl rapidly, following the copper plate. Faraday was the first to explain Arago's experiment.
FIG. 26–ARAGO'S EXPERIMENT