tion is universal for electromagnets today. Insulated wire had pre- 

 viously been applied to other uses, including galvanometer coils, hut 

 not to magnets. 



2. Proportioning of Magnet and Battery for Maximum Effect {1830) 

 Henry pointed out that the coils of the magnets constructed by his 

 method could be made up either of one very long wire or several shorter 

 ones, showed the proper relative proportioning of magnet and battery 

 for maximum effect in these two cases and pointed out that the intensity 

 magnet and battery were suitable for operating over line wires of 

 considerable resistance, while the quantity magnet and battery were 

 suitable for local circuits. This result was arrived at experimentally 

 before Henry had any knowledge of Ohm's law, and is the first recog- 

 nition of the necessity for properly proportioning the impedance of 

 different parts of an electric circuit which has become an important 

 quantitative requirement in the design of all forms of electric circuit, 

 particularly in communication circuits. 



3. Electromagnetic Motor (1831) 



Henry was the first to publish a description of an electric motor 

 using electromagnets and a commutator. This motor was a recipro- 

 cating device, therefore quite different in form from the commercial 

 motors developed later. It did, however, contribute these two funda- 

 mental elements of permanent value and also served to rouse interest 

 in and stimulate work on the problem of obtaining mechanical power 

 from electricity. 



The question has been raised whether Henry w^as not anticipated 

 by Salvatore dal Negro of Padua, who made a similar motor ap- 

 parently in the same year. While the evidence on this point is some- 

 what circumstantial, it seems to indicate clearly that Henry was first 

 in publication. It is probable that dal Negro's work was done without 

 knowledge of Henry's, but there is no evidence to indicate that his 

 motor was constructed earlier than Henry's. 



4. Telegraphy {1831 and 1832) 

 By the application of his principles of proper proportioning of mag- 

 net winding, battery and external circuit resistance, Henry produced 

 the first telegraph using electromagnets as a receiving device. The 

 idea of using galvanic current for telegraphy at a distance had pre- 

 viously been investigated by Barlow and later the control of electro- 

 magnets at a distance by transmitting current over a line wire was 

 investigated by W'heatstone without knowledge of Henr>''s work done 

 five years earlier. In both cases the project was rejected as impractic- 



