ii6 GEORGE HERBERT STOCKBRIDGE 



ing, as we shall see, the one feature which makes the electro- 

 magnet, as Vail left it, one of the happiest of modern inven- 

 tions, fitting it to be the hand of the far writer, the tongue of 

 the far speaker, the member which translates volition into 

 mechanical movement a hundred miles away, betrays the 

 guilty step of the burglar without his knowledge, and utters 

 the note of warning when a switch is misplaced or a dam 

 threatens to give away. 



Fortunately, Professor Henry's services to the telegraph 

 rest on a surer basis than the Albany trial system alone. 

 Morse, who knew nothing of any other electromagnet but 

 that of Sturgeon, was brought to a desperate standstill in his 

 search for the electric telegraph, by learning that he could 

 produce electromagnetic effects only through a short length 

 of wire. It was through Professor Gale's making him ac- 

 quainted with the Henry improvement that his despair was 

 changed to hope, and that the work was resumed. An indis- 

 pensable Unk in the telegraphic chain was wrought by Henry 

 in his development of the Sturgeon magnet. 



The other labors of Professor Henry are only less note- 

 worthy, because they happened to be concerned with matters 

 of less popular and industrial interest. His investigations in 

 magnetoelectric induction along the line of Faraday's work 

 are so important that an effort is now being made to secure 

 recognition for them by the general adoption of the term 

 "Henry" as the designation of the unit of electrical inductance. 

 In fact, Henry was within a few weeks of the great Faraday 

 in the discovery of the means for converting magnetism into 

 electricity — ''the greatest experimental result," says Tyndall, 

 "ever obtained by an investigator." 



He also went far towards anticipating Dr. Hertz and later 

 investigators, when he proposed the hypothesis of an ''elec- 

 trical plenum," to account for an inductive effect similar to 

 that which is utiUzed in telegraphing to and from railway 

 trains, where the results are brought about by the induction 

 of a current in a parallel circuit through wide spaces of air. 

 In a word, the contributions of Professor Henry to pure 

 science and to the electrical arts were many in number, and 



