THE HENRY. 145 



he presented to the French Academy a paper in which he announced 

 the fundamental principle of the science of electro-dynamics, together 

 with a number of capital exi)erimeuts iu extension of Oersted's princi- 

 ple, lu the incredibly short time of a single week he had gone all over 

 Oersted's work, experimentally and theoretically; he had devised a 

 new and ingenious hypothesis, for the examination of which he had 

 invented novel forms of apparatus and by means of which he had 

 brought the whole subject within the domain of mathematical treat- 

 ment. The history of the science of electricity shows nothing more 

 brilliant than the work of that inemorable week. To him who was first 

 to show the action and reaction of currents upon each other, and at 

 the same time furnish a rational and most useful hypothesis u})on which 

 the now rapidly growiug theory of electro-magnetism might be con- 

 structed, has long been freely accorded the high jnaise which is imi^lied 

 in calling the unit of current measure an ampere. 



The beautifully simple law of Ohm, to which reference has already 

 been made, and which is as omnipresent and omnipotent in electricity 

 as is N^ewton's law of gravitation in astronomy and mechanics, is 

 administered by and through a triumvirate. Two of the triad, immely, 

 resistance and. current, are presented above. The third, which is 

 mathematically the product of these two, is the electro-motive force 

 iu the circuit, and its unit of measure is the volt. The appropriatenes-s 

 of this luime will be at once recogui/.ed when the services of the dis- 

 tinguished Italian philosopher, Volta, the contemporary of Galvani, 

 are remembered. In his early youth Yolta was considered dull, and 

 he showed little promise of future distinction. His first awakening 

 to intellectual activity manifested itself in a tendency to compose 

 l)oetry, but from this he turned to exi)erimental science; and when 

 Galvani, in 1780, saw in the twitchings of the legs of a frog the begin- 

 ning of a series of marvelous discoveries which have nuide the nine- 

 teenth century greater than any that have gone before, Yolta was in 

 the prime of life, thoroughly equipped by taste and experience to take 

 up the subject at a point where his countryman seemed likely to leave 

 it, and so enlarge and enrich it as almost to make it entirely his own. 



Differing from Galvani as to the cause of what was long called 

 "galvanism,'' he originated what is known as the "contact tlieory," 

 and was the first to have clear ideas of what is now termed "electro- 

 motive force." His theory led him to the construction of the voltaic 

 ])ile or battery, which has been of incalculable value in the develop- 

 ment of the science of electricity and its applications. It happens 

 that the unit of measure, one volt, is very nearly the electro motive 

 force of one cell of Yolta's battery, being a little less than that of an 

 ordinary sulphate of copper ("bluestone") cell. 



These, the ohm, the ami)ere, and the volt, are the three fundamental 

 units of electrical measurement. They are related to one another 

 through Ohm's law, and, as other units are largely derived from them 

 SM 91 10 



