able because of their lack of appreciation of this principle of proper 

 proportioning. 



Henry's telegraph included an "intensity" battery, a line wire of 

 more than one mile in length and a receiving device consisting of an 

 electromagnet with polarized armature so arranged as to strike a bell 

 when reversals of current were sent. The use of the electromagnet 

 made rapid signaling possible and has formed the basis of practically 

 all subsequent commercial telegraph de\'elopment. He was also the 

 first to employ audible reception, which has played a very important 

 part, particularly in American telegraph practice. 



In Henry's later telegraph experiments he used a grounded circuit 

 which has played a very important part in telegraph development. 

 Grounded circuits had, however, been used before Henry in experi- 

 ments with electric telegraphs of other types. 



In 1850 in a IMorse patent suit a claim was made that in 1830 

 Abraham Booth from Dublin had demonstrated a similar electro- 

 magnetic telegraph. A search of important contemporary publica- 

 tions, however, reveals no evidence to support this claim. 



5. Electromagnetic Induction {1832) 



Faraday and Henry both independently discovered the phenomena 

 of mutual induction and of self induction. On the basis of date of 

 publication the discovery of mutual induction is properly credited to 

 Faraday (1831, published 1832) and the discovery of self induction 

 to Henry (1832). The determination of this question is recognized 

 in the official adoption by the International Electrical Congress of 1893 

 of the name "henry" for the practical unit of inductance. 



It is interesting to note that in 1804 Vassal i-Eandi observed that the 

 shock obtained in opening a long circuit was greater than that received 

 by merely touching the battery terminals. While this fact was due 

 to the self inductance of a circuit this observation did not constitute 

 an anticipation of Henry, as no thought of induction, either self or 

 mutual, was in the mind of the investigator. Also, in connection with 

 his investigations of mutual induction in 1831, Faraday conceived 

 that conductors might have the property of self inductance, and 

 carried on experiments designed to detect this property. His experi- 

 ments, however, showed a negative result. 



6. Electromagnetic Relay {1835) 



Henry first discovered and demonstrated the principle of the electro- 

 magnetic relay. The relay was used by him at the end of his telegraph 

 line to open the local circuit of a large magnet and to demonstrate 



