104 THE PATH OF SCIENCE 



were immediately follo^ved by those of a great Frenchman, 

 Andre Ampere, who wdthin a few months of Oersted's an- 

 nouncement found tliat two parallel electric currents would 

 behave like magnets, attracting and repelling each other, 

 according to the direction in which they flow^ed. In 1823 

 Ampere published a paper setting forth the mathematical 

 theory of the effects of electromagnetism. 



The inverse effect to Oersted's w^as not discovered for some 

 years. It was in 1831 that Michael Faraday found that the 

 movement of an electric circuit in a magnetic field caused a 

 current to start in the circuit, so that just as Oersted had 

 shown that a current produced a magnetic field, Faraday 

 showed that a magnetic field would produce a current. As 

 a result of this discovery, it became possible to generate elec- 

 tricity mechanically by the rotation of coils of wire in the 

 field of a magnet. This is the arrangement known as the 

 dynamo; and the inverse arrangement, in which the passage 

 of electricity through a coil in a magnetic field causes it to 

 rotate, is the electric motor. Thus, by the middle of the 

 nineteenth century, from the work of Ampere, Michael Fara- 

 day, and Joseph Henry in the United States, the general na- 

 ture of current electricity and especially the properties of 

 circuits carrying direct current were completely unraveled. 

 The development of alternating-current electricity belongs 

 to the field of engineering rather than of physics. It is the 

 work of the electrical engineers in the second half of the 

 nineteenth century that made possible the great use of elec- 

 tricity in practical applications. Electric light, the telegraph, 

 the telephone, and so forth represent applications of early 

 discoveries relating to electric currents by a large group of 

 scientists and an even greater group of engineers. 



The nature of electricity itself long remained completely 

 hidden. Its elucidation came not from further work in rela- 

 tion to electricity itself, but from the study of the conduction 

 of electricity through gases. As early as 1785, William 

 Morgan in a paper before the Royal Society referred to the 

 glow that could be obtained when electricity was passed 



