126 LEADING AMERICAN MEN OF SCIENCE 



well permit; and unless some essential improvements have lately 

 been made in these European plans, I should prefer the one in- 

 vented by yourself. 



"With my best wishes for your success, I remain with much 

 esteem, 



"Yours truly, 



"JOSEPH HENRY." 



It was two years after the date of this letter in May, 1844, that 

 the first telegraphic message was transmitted from Washington 

 to Baltimore. 



In 1831 he made what was probably the first observation of a 

 magnetic storm in this country. This term is applied to very 

 small changes in the direction in which a magnet points, and 

 in the force which the earth produces upon it, that occur from 

 time to time. These disturbances of the magnetic needle are 

 called "storms" because they behave much like a storm of 

 wind in moving the magnet about. On the same evening in 

 which the storm was first noticed a brilliant aurora commenced. 

 It has since been found that unusual displays of the aurora are 

 nearly always accompanied by magnetic storms. 



The next discovery of Henry was one in which, although it 

 was quite original, he was anticipated in publication by Faraday. 

 This was the production of magneto electricity. When it was 

 known that electricity could make iron into a magnet in the way 

 I have described, the idea naturally occurred that, conversely, 

 magnets might also produce electricity. Efforts to produce elec- 

 tricity in this way were unavailing until Henry showed that the 

 mere presence of a magnet was not sufficient, but that the magnet 

 must move. Henry's discovery may be explained in the follow- 

 ing way. Let us suppose a long piece of wire wound round and 

 round in a coil, like a coil of rope, but without anything inside of 

 it. Then bring the two ends of the wire into contact. Of 

 course this alone would be nothing but a commonplace coil of 

 wire. Now take a powerful magnet and insert it inside the 

 coil. While you are doing this an electric current will pass 

 through the coil, but the moment you get the magnet inside and 



