Maech 18, 1898.] 



SCIENCE. 



365 



this time the important discovery that if a 

 needle be suspended above a copper disc 

 and the disc rotated the needle will be 

 dragged round with the disc. This was not 

 explained for some years, but seems to be 

 the first discovery of induced currents. 



These experiments mark the discoverj' of 

 electro-magnetism, and began one of the 

 most important eras in electrical discovery, 

 the work which has been participated in 

 by many eminent authorities. Among the 

 many advances may be mentioned the ex- 

 periments of Henry on the relative effects 

 of different windiugs on the strength of an 

 electro-magnet. He deduced the fact that 

 the magnetizing action might be increased 

 either by increasing the number of windings, 

 the current remaining the same, or by in- 

 creasing the current, the winding remaining 

 the same. He pointed out the application 

 of this to intensity and quantity arrange- 

 ments of the battery, and also the impor- 

 tance of the intensity winding for the trans- 

 mission of magnetizing power to a distance, 

 as in telegraphy. The increased eifect due 

 to increasing the number of windings on 

 the coil of a galvanoscope had been pre- 

 viously pointed out by Schweigger, and the 

 •discovery is embodied in Sohweigger's gal- 

 vanoscope. 



In 1821 Faraday began his researches 

 and many important discoveries were made 

 by him. The main guiding idea in Fara- 

 day's work was the possibility of obtaining 

 electricity from magnetism and in general 

 the discovery of the inter-relation between 

 the two. In this connection Arago's dis- 

 covery of the rotation of a copper disc by 

 the rotation of a magnet above it is of great 

 importance, because, among other things, 

 Faraday set himself to explain this. The 

 result was the discovery of the commutator- 

 less dynamo, or Faraday disc. In view of 

 modern developments, probably the most 

 important of Faraday's discoveries was that 

 of the production of a current in a circuit 



when a current is either established or 

 varied in strength in an adjacent circuit. 

 This was followed by the discovery that rel- 

 ative motion of two circuits, one of which 

 carried a current produced a current in the 

 other, and that the motion of a magnet in 

 the neighborhood of a circuit produced a 

 current in the circuit. Another important 

 discovery by Faraday was that of the 

 quantitative laws which govern electrolytic 

 decomposition, thus giving us our electro- 

 chemical equivalents. 



At this time Lenz was led by experi- 

 ment to the discovery of his celebrated law 

 of induction, namely, that the current pro- 

 duced always in turn produces forces tend- 

 ing to oppose the change. For example, if 

 a cu^rrent be induced in a coil by bringing 

 a magnet towards it the mutual action be- 

 tween the magnet and the current is to op- 

 pose the magnet's approach. This is im- 

 portant when looked at from the point of 

 view of the conservation of energy or as 

 an argument against perpetual motion. 

 Lenz's law is, of course, when the actions 

 are properly understood, a consequence of 

 Newton's third law of motion. 



Discoveries similar to those of Faraday 

 as to induced currents were made almost 

 simultaneously by Henry in this country. 

 We have in the discoveries of Faraday and 

 Henry the fundamental information re- 

 quired for nearly the whole of our recent 

 developments in dynamo-electric generators 

 and electric motors, but it was reserved for 

 the next generation to develop them. This 

 development we owe in no small degree to 

 the spendid exposition of Faraday's dis- 

 coveries and their consequences contained 

 in Maxwell's book on electricity and mag- 

 netism. 



Going back for a moment to 1822 we 

 have to notice another important discovery, 

 namely, the thermoelectric couple by See- 

 beck. There followed almost immediately 

 the important experiment of Gumming, who 



