360 PIONEERS OF SCIENCE IN AMERICA. 



Silliman. It consisted of an octagonal bar three inches thick 

 and thirty inches long, bent in the form of a horseshoe, and 

 was able to lift more than a ton. For his own laboratory at 

 Princeton be constructed one to surpass it. This lifted more 

 than three thousand pounds. One other contribution Henry 

 here made to the telegraph. He used the intensity spool and 

 battery, working through long distances, to open and close the 

 circuit of the quantity spool and battery, . . . thus making the 

 powerful magnet at short range the servant of the weak one 

 at long range. This device of opening one circuit by means 

 of another is used in the relay to call into play another inten- 

 sity circuit. 



When he had been four years at Princeton he was given a 

 year's leave of absence on full salary, in which he made a very 

 enjoyable visit to Europe. He and his friend Dallas Bache 

 arrived in England in February, 1837. He received an en- 

 thusiastic welcome from the scientists of the old world, espe- 

 cially from Faraday. The importance of his visit to London 

 to the telegraph in England should not be overlooked. Wheat- 

 stone was then busy with his form of the invention. He had 

 discarded the electro-magnet, because, to use his own words, 

 " sufficient attractive power could not be imparted to an 

 electro-magnet interposed in a long circuit," and had substi- 

 tuted for it a secondary galvanic circuit. Henry at this time 

 explained to him his discovery of the intensity magnet. Wheat- 

 stone never acknowledged any indebtedness to Henry, but it 

 is a significant fact that before Henry's arrival in England the 

 electro-magnet was discarded as inefficient by Wheatstone, and 

 after two weeks of daily intercourse with Henry it was re- 

 stored to his telegraph, and the success of the English system 

 secured. Crossing over to Paris Henry made the acquaintance 

 of Arago, Becquerel, De la Rive, Biot, and Gay Lussac. A 

 visit to Edinburgh brought him into contact with other con- 

 genial minds, and in September he attended the meeting of the 

 British Association, which he addressed, by invitation, giving 

 some of his recent observations on the " lateral discharge " 

 from the Leyden jar or a conducting wire. He also gave the 

 section of Mechanics an account of the great extension of the 

 railway and canal systems in the United States. Returning 

 home in November, greatly invigorated by his tour, Prof. 

 Henry plunged again into his fruitful labours. 



