HENRY AS AN ELECTRICAL PIONEER 329 



Henry produced currents in distant circuits by oscillatory discharges 

 and detected them when the two circuits were separated by several 

 hundred feet. (1842) 



These contributions by Henry to our electrical knowledge have been 

 essential to the development of practically every commercial applica- 

 tion of electricity. Let us now consider some of these. 



The idea of the electric telegraph was not new when Henry did his 

 work. It had been considered by others and in January, 1825, Barlow, 

 an English scientist, wrote as follows: " In a very early stage of electro- 

 magnetic experiments, it had been suggested, that an instantaneous 

 telegraph might be established by means of conducting wires and 

 compasses. The details of this contrivance are so obvious, and the 

 principles on which it is founded so well understood, that there was 

 only one question which could render the result doubtful, and this was, 

 Is there any diminution of effect by lengthening the conducting wire? 

 ... I was, therefore, induced to make the trial, but I found such a 

 sensible diminution with only 200 feet of wire, as at once to convince 

 me of the impracticability of the scheme." ^ Even as late as 1837 

 Wheatstone, another distinguished British scientist, satisfied himself 

 by experiment and convinced others that the development of electro- 

 magnetism in soft iron at a distance was impracticable. This was 

 six years after Henry's work on the proportioning of magnet windings 

 and battery arrangement for maximum effect with line wires of sub- 

 stantial resistance had shown him the solution of the telegraph prob- 

 lem, and five years after he himself had constructed and demonstrated 

 his electromagnetic telegraph over a line wire more than a mile long. 



Henry's contributions to the telegraph were three in number. He 



demonstrated that magnetic action and magnetic control could be 



exercised at considerable distances if the battery and the magnet 



windings were suitably proportioned and this is the basis of all electric 



telegraphs of today. He abandoned the galvanometer or compass 



needle as a receiving device for the electrical impulses and substituted 



therefor a magnet operating a movable armature, this making possible 



rapid signaling and audible receiving. This has continued as the 



basic form of the telegraph circuit, even with the modern printing 



telegraph systems, in which electric typewriters are controlled by 



magnets of Henry's type. He constructed and operated the first 



electromagnetic relay, a device by which the current in the line circuit 



controls an armature which carries the contact of a local circuit so 



that the feeble line current, instead of directly controlling the receiving 



2 "On the laws of electro-magnetic action," Edinburgh Philosophical Journal. 

 Jan., 1825, Vol. XII, p. 105. 



