THE GROWTH OF PHYSICAL IDEAS 103 



signed, and the Leyden jar was invented, in which two con- 

 ducting layers were separated by the glass of the jar, so that 

 opposite charges could be stored on the t^vo faces of the glass. 

 Interest in static electricity was greatly stimulated by the spec- 

 tacular results obtained. Benjamin Franklin discovered that 

 the charges in the jar reside on the glass walls, and he built 

 a condenser using a series of glass plates separated by sheets 

 of tin foil, thus obtaining the condenser which we use today. 



All this work dealt wdth static electricity, and it \\as not 

 until the close of the eighteenth century that electricity in 

 motion was investigated. Luigi Galvani, an Italian anato- 

 mist, observed that under the stimulation of an electric 

 charge a frog's legs isolated from the body would show con- 

 traction and that it could be produced by the simple contact 

 of two different metals moistened by the salty juices of a 

 frog's body. Galvani, in fact, discovered the possibility of 

 producing an electric current, for which the frog's leg was 

 the detector. This discovery was followed up by Alessandro 

 Volta, who in 1800 announced his voltaic pile, which con- 

 sisted of a series of alternate copper and zinc plates sepa- 

 rated by pieces of paper or flannel moistened with brine. 

 This was the first battery, and experimenters soon designed 

 improved batteries and were able to get electric currents with 

 which chemical effects could be produced. Water was de- 

 composed into hydrogen and oxygen, and Davy, experiment- 

 ing at the Royal Institution, decomposed potash wdth a bat- 

 tery of two hundred and fifty cells and obtained the metal 

 potassium. Later he prepared sodium, calcium, barium, 

 strontium, and magnesium. With two thousand cells, he 

 produced the first arc lamp and in the arc melted refractory 

 substances such as platinum and quartz. 



In 1819, Hans Oersted of the University of Copenhagen 

 made a discovery which is the foundation of the science of 

 electromagnetism. He found that a wire carrying a current 

 would displace a compass needle w^hen it was parallel to it 

 and thus demonstrated that a conductor bearing an electric 

 current produces a magnetic field. Oersted's experiments 



