342 A CENTURY OF SCIENCE 



instead of the short wire, though no spark will be per- 

 ceptible when the connection is made, yet when it is 

 broken by drawing one end of the wire from its cup of 

 mercury a vivid spark is produced. . . . The effect 

 appears somewhat increased by coiling the wire into a 

 helix ; it seems to depend in some measure on the length 

 and thickness of the wire ; I can account for these phe- 

 nomena only by supposing the long wire to become 

 charged with electricity which by its reaction on itself 

 projects a spark when the connection is broken." 



Soon after, Henry went to Princeton and there con- 

 tinued his experiments in electromagnetism. No diffi- 

 culty was experienced in inducing currents of the third, 

 fourth and fifth orders by using the first secondary as 

 primary for yet another secondary circuit, and so on 

 (38, 209, 1840). The directions of these currents of 

 higher orders when the primary is made or broken 

 proved puzzling at first, but were satisfactorily explained 

 a year later (41, 117, 1841). In addition induced cur- 

 rents were obtained from a Leyden jar discharge. Fara- 

 day failed to find any screening effect of a conducting 

 cylinder placed around the primary and inside the 

 secondary. Henry examined the matter, and found that 

 the screening effect exists only when the induced current 

 is due to a make or break of the primary circuit, and not 

 when it is caused by motion of the primary. 



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 



