244 MEMORIAL OF JOSEPH HENRY. 



"quantity" magnet of any practicable power: a work which 

 indeed could be accomplished by the mere swing of the most deli- 

 cate galvanometer needle. Professor Henry had constructed for 

 his own laboratory a large electro-magnet designed to surpass the 

 celebrated magnet made for Yale College; and with it he was ena- 

 bled to exhibit to his class, by employing a small portion of his 

 "quantity" battery, an easy lifting power of more than three thou- 

 sand pounds.* Such 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. This 

 combination has since found an important application; its principle 

 underlying all the various forms and uses of the "relay" magnet, 

 and of the "receiving" magnet and local battery, since employed. 



Visit to Europe. In order to give Professor Henry a much- 

 needed rest from his diligent services and close application during 

 the last 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 Avith his valued and faithful 

 friend, Professor Bache, 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 memories of their holiday sojourn abroad. 



In London, many pleasant interviews with Faraday, formed a 

 memorable circumstance. Wheatstone, then Professor of Experi- 

 mental Philosophy in King's College, was engaged in developing his 

 system of needle telegraph, and he unfolded freely to his visitors 

 his numerous projects; and particularly his arrangement of sup- 

 plementary local circuit from an additional battery, for sounding 

 an electro-magnetic signal, by being brought into action by a move- 

 ment from the main line circuit, f Henry had then the pleasure 



* It is said that this magnet has been made to sustain 3,500 pounds. (Turnbull's 

 Electro-Magnetic Telegraph, 2nd ed. 1833, p 49.) 



t This was early in April, 1837. (Smithsonian Report for 1857, p. 111.) Two months 

 later, or June 12th, 1837, WHEATSTONE in conjunction with W. F. COOKE had secured 

 a patent on his system of telegraph, including the combination of circuits. 



