and also from experinieiU of paragraph 72 the arinature itself may circulate a closed 

 current which will interfere with the intensity of the induction in the surrounding 

 wire. I am inclined to believe that the increased effect observed by Sturgeon and 

 ("allaiid, when a bundle of v\ ire is substituted for a solid [)iece of iron, is at least in part 

 due to the interrui)tion of these currents. I hope to resume this [lart of the subject, 

 in connection with se\eral other i)oints, in another communication to the Society." •** 



At the end of the meeting in which this communication was presented, 

 Dr. Bache gave an account of investigations of Professor Ettingshausen 

 of X'ienna, in which he had been led to suspect the development of a 

 current in the metal of the keeper of the magneto-electrical machine, 

 which diminished the effect of the current in the coil about the keeper. 

 Mentioning this communication, Henry said: 



"It gives me pleasure to learn that the improvements, which I have merely sug- 

 gested as deductions from the principles of the interference of induced currents (76), 

 should be in accordance with the experimental conclusions of the above named phil- 

 osopher." ^'^ 



Under date of November 13, 1838, a few days after Henry's com- 

 munication to the American Philosophical Society, a communication 

 bearing on this subject was sent to Sillimans Journal by Charles G. 

 Page and published in that Journal in January, 1839. Mr. Page 

 states : 



"... I would not say that a very careful insulation [of the iron wires forming a 

 magnet core] might not improve their operation. For I apprehend that in the devel- 

 opment and return of magnetic forces, electrical currents are excited in the body of 

 the magnet at right angles to its axis, as well as in the wires surrounding the magnet." 



The work of Foucault, whose name is often associated with this type of 

 induced current, was published in Comptes Renins, 1855. Foucault 

 was familiar with the work of Arago and Faraday, but did not mention 

 the work of Henry. 



10. Discoveries Bearing Particularly on Radio Telegraphy 

 and Telephony 



a. Variation of Induction Between Coils with Separation 



Henry, in his extensive investigations of induction, as presented in a 

 paper in 1838 and published in 1839," showed that the current induced 

 in the secondary of two helical coils could be varied or regulated by 

 changing the relation in space of the coils. The following from several 

 of Henry's applications of this principle gives a partial summary of the 

 results of his investigations: 



"The action at a distance afifords a simple method of graduating the intensity of 

 the shock in the case of its application to medical purposes. The helix may be sus- 



^ SW, Vol. 1, p. 126. 

 ^-^SW, Vol. 1, p. 145. 

 "SW, Vol. 1, pp. 119-122. 



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