MAGNETISM AND ELECTRICITY 207 



should now call Volta's "crown of cups" a group of batteries 

 connected in series, as will be explained later. The electricity 

 generated by such means came to be known as galvanic electricity 

 from its discoverer, Galvani, who, however, misunderstood its 

 source. Galvanic and frictional electricity are identical. 



One day in 1819 when Hans Oersted, professor of physics in 

 the University of Copenhagen, Denmark, was working with 

 electric currents he noticed that a compass needle that happened 

 to be standing on the table moved every time an electric current 

 was sent through a wire near it. He began to investigate, and 

 found (1820) that, when a wire is held over or under the 

 magnetic needle and parallel to it, and an electric current is sent 

 through the wire, the needle turns and tends to stand at right 

 angles to the wire. If the current is strong it will assume such 

 a position ancl keep it while the current is maintained. 



Andre Ampere, who was a professor at the Polytechnic School 

 in Paris, heard of this law that Oersted had discovered. He 

 repeated the experiments, verified Oersted's results, but found 

 out something more. He noticed that when the wire was held 

 over the needle the north pole was deflected in one direction, but 

 when held under the needle it turned in the opposite way. 

 Furthermore, if the current in the wire held over the needle 

 was going in one direction, the north pole of the needle was 

 deflected one way, but if the current was reversed the north pole 

 swung in the opposite direction. This law may now be stated 

 thus: If you imagine yourself swimming, breast toward the 

 needle, along the wire in the direction the current is going, the 

 north pole of the needle will swing to your left. You may readily 

 try this experiment for yourself with a compass and a copper wire, 

 the ends of which connect with the binding-posts of an ordinary 

 dry battery. The current is said to flow through the wire from 

 the carbon at the center of the dry cell to the zinc at its edge. 



Ampere applied this knowledge he had discovered to the con- 

 struction of an instrument for detecting electric currents. A 

 compass was wound with many turns of insulated copper wire 



