310 L. Page — A Century's Progress in Physics. 



Henry's work was mainly descriptive ; it remained for 

 Faraday to develop a theory to account for the phenomena 

 discovered and to prepare the way for quantitative for- 

 mulation of the laws of current induction, This he did in 

 his representation of a magnetic field by means of lines 

 of force ; a conception which he found afterwards to be 

 equally valuable when applied to electrostatic problems. 

 Every magnet and every current gives rise to these 

 closed curves ; in the case of a magnet they thread it 

 from south pole to north, while a straight wire bearing 

 a current is surrounded by concentric rings. The con- 

 nection between lines of force and the induction of cur- 

 rents is contained in the rule that a current is induced in 

 a closed circuit only when a change takes place in the 

 number of lines of force passing through it. Further- 

 more the dependence of the current strength on the 

 conductivity of the wire employed has led to recognition 

 of the fact that it is the electromotive force and not the 

 current itself which is conditioned by the change in mag- 

 netic flux. 



Great interest was attached to the utilization of the 

 newly discovered forces of electromagnetism. In 1831 

 Henry (20, 340, 1831) described a reciprocating engine 

 depending on magnetic attraction and repulsion, and C. 

 G. Page (33, 118, 1838; 49, 131, 1845) devised many 

 others. The latter 's most important work, however, was 

 the invention of the RuhmkorfT coil. In 1836 (31, 137, 

 1837) he found the strongest shocks to be obtained from a 

 secondary coil of many windings forming a continuation 

 of a primary of half the number of turns. His perfec- 

 tion of the self-acting circuit breaker (35, 252, 1839) 

 widened the usefulness of the induction coil, and his sub- 

 stitution of a bundle of iron wires for a solid iron core 

 (34, 163, 1838) greatly increased its efficiency. 



Conservation of Energy. — Perhaps the most important 

 advance of the nineteenth century has been the estab- 

 lishment of the principle of conservation of energy. 

 Despite the fact that the "principe de la conservation des 

 force vives" had been recognized by the French mathe- 

 maticians of the early part of the century, the application 

 of this principle even to purely mechanical problems was 

 contested by some scientists. Through the early num- 

 bers of this Journal runs a lively controversy as to 

 whether there is not a loss of power involved in impart- 



