It appears that the experimenter observed the phenomenon of mo- 

 mentary high voltage produced by the "extra current" later ob- 

 served by Henry, but that it was in no sense an anticipation of the 

 discovery of self-induction since it was not so interpreted, no thought 

 of induction, self or mutual, being in the mind of the experimenter. 



Henry's first publication on mutual induction was in Silliman's 

 Journal, July, 1832. He mentioned having seen a short account of 

 Faraday's work and the method he used, saying: 



"Before having any knowledge of the method given in the above account, I had 

 succeeded in producing electrical effects in the following manner, which differs from 

 that employed by Mr. Faraday, and which appears to me to develop some new and 

 interesting facts." ^' 



He then described his experiment, which consisted in opening and 

 closing the circuit of an electro-magnet and noting the effect on a galva- 

 nometer connected to a coil wound around the armature of the magnet. 



Faraday read a detailed paper November 24, 1831, on mutual induc- 

 tion, this appearing in Philosophical Transactions, 1832. The account 

 Henry saw was the proceedings of the Royal Institution as reported 

 in the Philosophical Magazine for April 1832, as follows: 



"Feb. 17. — Mr. Faraday gave an account of the first two parts of his recent re- 

 searches in electricity; namely, volta-electric induction, and magneto-electric 

 induction. 



If two wires, A and B, be placed side by side, but not in contact, and a voltaic 

 current be passed through A, there is instantly a current produced, by induction, in B, 

 in the opposite direction. ... If a wire connected at both extremities with a gal- 

 vanometer be coiled, in the form of a helix, around a magnet, no current of electricity 

 takes place in it. . . . But if the magnet be withdrawn from or introduced into such a 

 helix, a current of electricity is produced rohilst the magnet is in motion, and is rendered 

 evident by the deflection of the galvanometer. . . . Thus is obtained the result so 

 long sought after, — the conversion of magnetism into electricity." - 



6. Electromagnetic Relay 



The date of Henry's first electro-magnetic relay is generally given 



as 1835. It was made at Princeton (where he went towards the end 



of 1832) after he became settled in his duties there, and before he went 



to England in 1837. Edward N. Dickerson,^^ a Princeton student in 



1839, said that Henry put his relay in operation at Princeton in 1835. 



Henry's earliest description of the relay, from a deposition of 1849, is as 



follows : 



"In February, 1837, I went to Europe; and early in April of that year Prof. 

 Wheatstone, of London, in the course of a visit to him in King's College, London, 

 with Prof. Bache, now of the Coast Surrey, explained to us his plans of an electro- 

 magnetic telegraph; and, among other things, exhibited to us his method of brmgmg 



21 SW, Vol. 1, p. 75. 



"^"■Philosophical Magazine, April, 1832, pp. 300-301. 



" Joseph Henry and the Atagnetic Telegraph: An Address by Edward N. Dicker son, 

 LL.D., 1885. 



15 



