144 THE HENRY. , 



passajie of the current by the increased length. Biulow suggested 

 that this couhl be overcome only by enlarging the dimensions of the 

 conductor, and that when a current was transmitted through any con- 

 siderable distance the diameter of the wire must be enormous. For this 

 reason the electro-magnetic telegraph was an impracticable scheme. 

 This apparently conclusive argument undoubtedly seriously delayed 

 the progress of invention along that line. 



But, curiously enough, about the time Ohm in Europe was publish- 

 ing a theoretical investigation which might have furnisluHl the key to 

 the solution of the problem, in Anu-rica a young man, not yet 30 years 

 of age, named Joseph Henry, had begun a series of experimental 

 researches at Albany, N. Y., which did make the way entirely clear a 

 few years later. Henry attacked the dilticulty both as to cause and 

 effect. The effect was that when the conductor through which- the 

 current Avas passing was increased greatly in length, the strength of 

 the current was so reduced that it was insufficient to operate the appa- 

 ratus necessary for the reproduction of the signal at the receiving end. 

 To nu-et this difficulty he investigated electro-magnets, and so improved 

 upon the original device of Sturgeon that comparatively feeble cur- 

 rents were capable of ])roducing mechanical efi^cts through long Avires. 

 He also originated the ingenious device known as a "relay," by means 

 of which a h)cal battery is put in operation by a main current of little 

 strength, thus making local effects independent of the strength of the 

 main-line currents. By his invention of the "intensity magnet" and 

 use of the "intensity battery" he nuide the electro-magnetic telegraph 

 possible, and in 1831 he transmitted signals through a mile of wire, 

 causing a bell to ring by the action of an electro-magnet. Out of this 

 has grown the astounding network of wires— overhead, underground, 

 and across the seas— by which the earth is girdled and the existence 

 of which has wrought more change in the treatment of social, political, 

 and commercial problems than any other single fact of tlie present 

 century. While many of the conclusions which Henry had experi- 

 mentally reached were in harmony with and might have been deduced 

 from Ohm's law, to Ohm belongs the credit of having first clearly 

 pointed out the real and exact meaning of "resistance" and its relation 

 to the other conditions of the circuit. The bestowal of his name upon 

 the unit by wiiich it is measured is a litting recognition of the lasting 

 value of his discovery. 



The ampere is the unit of current. Andre Marie Ampere, born at 

 Lyons, France, in 1775, must be regarded as the creator of the science 

 of electro-dynamics. In 1820 Oersted, the Dane, publislied his mag- 

 niticent discovery of the effect of an electric- current upon a freely sus- 

 pended magnet, thus establishing the relation between magnetism and 

 electricity which many of the ablest philosophers had sought in vain 

 for years. Ampere first heard of what was culled the "Copenhagen 

 experiment" on September 11, 1820. On the 18th of the same month 



