A CENTURY'S PROGRESS IN PHYSICS 343 



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 Ruhmkorff 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 the Journal runs a lively controversy as to 

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

 ing momentum to the reciprocating parts of a steam 

 engine only to check the motion later on in the stroke. 

 Finally Isaac Doolittle (14, 60, 1828), of the Bennington 

 Iron Works, ends the discussion by the pertinent remark : 

 "If there be, as is contended by one of your correspond- 

 ents, a loss of more than one third of the power, in trans- 

 forming an alternating rectilinear movement into a 

 continuous circular one by means of a crank, I should 

 like to be informed what would be the effect if the propo- 

 sition were reversed, as in the case of the common 

 saw mill, and in many other instances in practical 

 mechanics. ' ' 



A realization of the equivalence of heat and mechani- 

 cal work did not come until the middle of the century, in 



