260 BULLETIN OF THE 



It is simply necessary to employ with the distant "intensity" 

 magnet an oscillating armature with a suitable prolongation 

 so arranged as to open and close the short circuit of an ad- 

 joining " quantity" magnet of any practicable power : — a work 

 which indeed could be accomplished by the mere swing of the 

 most delicate galvanometer needle. Professor Henry had con- 

 structed for his own laboratory a large electro-magnet designed 

 to surpass the celebrated magnet made foi i'ale College; and 

 with it he was enabled to exhibit to his class, by employing a 

 small portion of his " quantity" battery, an easy lifting power 

 of more than three thousand pounds.* Sucli was the mechanical 

 agency he called into action through his telegraphic circuit, by 

 simply lifting its galvanic wire from a mercury thimble, or by 

 again dipping it into the same. Although this special com- 

 bination has not found any important application, its principle 

 underlies all the various forms and uses of the "relay" magnet 

 and local battery since employed. 



Visit to Europe. — In order to give Professor Henry a rauch- 

 oeeded rest from his diligent services and close application of 

 the past four years, the Trustees of his College liberally allowed 

 him a year's absence with full salary : thus affording him for 

 the first time a long coveted opportunity of visiting Europe. 



In February of 1837, in company with his valued and faithful 

 friend. Professor Bach^, he arrived in England ; where the two 

 American physicists formed ready and lasting intimacies with 

 some of the most distinguished worthies of Great Britain. 

 Everywhere received with courteous and cordial consideration, 

 they both ever carried with them agreeable memoi'ies of their 

 holiday sojourn abroad. 



In London, many pleasant interviews with Faraday, formed o, 

 memorable circumstance. Wheatstone, then Professor of Ex- 

 perimental Philosophy in King's College, was engaged in de- 

 veloping his system of needle telegraph : and Henry had the 

 satisfaction of finding that his own early investigations were 

 recognized and appreciated, and their results successfully 

 adopted. Wheatstone unfolded freely to his visitors his numerous 

 projects ; and particularly his arrangement of supplementary 

 local circuit from an additional battery, for sounding an electro- 

 magnetic signal, by being brought into action by a movement 

 from the main line circuit. f Henry had then the pleasure of 

 detailing to him his own similar combination of two electro- 



* It is saifi that this mas;net has been maile to sustain 3.500 pounds. 



t This was early in April, 1837. (Smithsonwn Report for 1857, p. 11].) 

 Two rnoiiths lat<^r, or .June 12tli, 1837, Wheatstone in conjunction with W. 

 F. Cooke iiad secured a patent on his system of telegraph, including the 

 ooinbination of circuits. 



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