744 THE POPULAR SCIENCE MONTHLY, 



development for which we might otherwise have had long to wait, so 

 now the requirements of electric lighting are giving rise to a new de- 

 velopment of the art of measurement upon a large scale, which can 

 not 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 surprising 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 amperes. 



The powerful currents which are now at command constitute al- 

 most 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, Fara- 

 day'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 scien- 

 tific 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 produced upon a scale that would have delighted their dis- 

 coverer, a rotation of the plane of polarization through 180° being per- 

 fectly feasible. With the aid of modern appliances, Kundt and Ront- 

 gen, in Germany, and H. Becquerel, in France, have detected the 

 rotation in gases and vapors, where, on account of its extreme small- 

 ness, 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 dynamo-ina- 

 chine the iron of the field-magnets approaches a state of saturation, 

 and the very elements of an explanation of the action require us to 

 take the fact into account. It is, indeed, probable that a better 

 knowledge of this subject might lead to improvements in the design 

 of these machines. 



Notwithstanding the important work of Rowland and Stoletow, the 

 whole theory of the behavior of soft iron under varying magnetic con- 

 ditions is still somewhat obscure. Much may be hoped from the in- 

 duction-balance of Hughes, by which the marvelous powers of the 

 telephone are applied to the discrimination of the properties of metals, 

 as regards magnetism and electric conductivity. 



The introduction of powerful alternate-current in machines by 

 Siemens, Gordon, Ferranti, and others, is likely also to have a salutary 

 effect in educating those so-called practical electricians whose ideas do 

 not easily rise above ohms and volts. It has long been known that, 



