94 RayleigWs Address to the British Association. 



careful consideration, but it is one worthy of an intelligent and 

 enterprising community. 



The requirements of practice react in the most healthy manner 

 upon scientific electricity. Just as in former days the science 

 received a stimulus from the application to telegraphy, under 

 which everything relating to measurement on a small scale 

 acquired an importance and development for which we might 

 otherwise have had long to wait, so now the requirements of 

 electric lighting are giving rise to a new development of the art 

 of measurement upon a large scale, which cannot fail to prove of 

 scientific as well as practical importance. Mere change of scale 

 may not at first appear a very important matter, but it is sur- 

 prising how much modification it entails in the instruments, and 

 in the processes of measurement. For instance, the resistance 

 coils on which the electrician relies in dealing with currents whose 

 maximum is a fraction of an ampere, fail altogether when it 

 becomes a question of hundreds, not to say thousands, of ampers. 



The powerful currents, which are now at command, constitute 

 almost a new weapon in the hands of the physicist. Effects, 

 which in old days were rare and difiicult of observation, may now 

 be produced at will on the most conspicuous scale. Consider for 

 a moment Faraday's great discovery of the "Magnetization of 

 Light," which Tyndall likens to the Weisshorn among mountains, 

 as high, beautiful, and alone. This judgment (in which I fully 

 concur) relates to the scientific aspect of the discovery, for to the 

 eye of sense nothing could have been more insignificant. It is 

 even possible that it might have eluded altogether the penetration 

 of Faraday, had he not been provided with a special quality of 

 very heavy glass. At the present day these effects may be pro- 

 duced upon a scale that would have delighted their discoverer, a 

 rotation of the plane of polarization through 180 ° being perfectly 

 feasible. With the aid of modern appliances Kundt and Rontgen 

 in Germany, and H. Becquerel in France, have detected the rota- 

 tion in gases and vapours, where, on account of its extreme 

 smallness, it had previously escaped notice. 



Again, the question of the magnetic saturation of iron has now 

 an importance entirely beyond what it possessed at the time of 

 Joule's early observations. Then it required special arrangements 

 purposely contrived to bring it into prominence. Now in every 



