JOSEPH HENRY 9 



vivid spark was produced, accompanied by a loud snap. The same 

 ribbon uncoiled gave a feeble spark." 



Henry tried many modifications of this experiment and in the end 

 drew the conclusion that the after-current he was observing was due 

 to the inductive effect of the current in the wire upon itself, and that 

 this became particularly apparent when the wire was so coiled that 

 its various turns lay close together. The discovery of mutual in- 

 duction by Faraday and the discovery of self-induction by Henry 

 constitute two halves of a whole, and it is appropriate that to these 

 men should go equal recognition in the matter of having electrical units 

 names after them. Of the three units by which the properties of every 

 electric circuit are measured, the unit of capacity was named after 

 Faraday, and unit of inductance after Henry; the third unit, that of 

 resistance, recognizes the fundamental researches of Ohm. 



A few years later, after having accepted the chair of physics at 

 Princeton University, Henry returned to the subject of induced cur- 

 rents. In his earlier work he, like Faraday, had used the continuous 

 currents which a voltaic battery generates. He now chose the cur- 

 rents which flow when a Leyden jar is discharged. To register the 

 inductive effects of the fleeting currents of discharge Henry adopted 

 a device consisting of an unmagnetized needle placed in a small coil 

 of wire. Through this coil the induced current had to flow. The 

 use of the needle as an indicator led Henry to an important observa- 

 tion. He noticed that following a discharge, the direction of magnetiza- 

 tion of the needle depended upon the distance across which the induc- 

 tive effect had occurred. To account for this curious result, he ad- 

 vanced the hypothesis — later shown to be correct — that the discharge 

 is oscillatory. 



Here was the germ of a great discovery. The oscillatory character 

 of the discharge is one of the fundamental and important properties 

 of certain types of electric circuit. Henry did not have the facilities, 

 however, for carrying his investigations in this field far enough to 

 attract the attention of the scientific world. It was not until 1835, 

 some thirteen years later, when Lord Kelvin was led independentl\- 

 by mathematical considerations to believe that the discharge is 

 oscillatory, that the significance of the phenomenon began to be 

 understood. 



Henry's work contained the germ of yet another important dis- 

 covery. Some of his experiments on induction by Leyden jar dis- 

 charges involved the transmission of electric force without wires 

 through distances as great as two hundred feet, and through the 

 floors and walls of buildings. And in similar experiments in which he 



