PHYSICS. 



By George F. Baekee, M. D., 



Professor of Physics in the University of Pennsylvania, Philadelphia. 



GENERAL. 



The address of Lord Eayleigh, president of the British Association, 

 at the Montreal meeting, reviewed very ably the recent progress of 

 physical science. One of the most striking advances is in the produc- 

 tion and application of electricity upon a large scale, the reaction of 

 which upon the advancement of pure science has been most salutary. 

 The electro-magnetic rotation of the polarized ray, which Faraday used 

 the most refined means to detect, can now be produced through 180° ; 

 the question of magnetic saturation, in the hands of Joule a purely sci- 

 entific one, is now a fundamental one in every dynamo macliine. The 

 laws of alternating currents show some curious modifications by induc- 

 tion in powerful machines. Suppose an electro-magnet wound with 

 two i^arallel wires, through one of which an alternating current passes 

 and develops a certain amount of heat in the circuit. If now the sec- 

 ond wire receive the current, it would at first be supposed that the heat 

 effect would be doubled. In fact, however, the total current, since it 

 is governed by the self-induction of the circuit, would not be increased 

 at all; and hence the heating effect would be actually halved in the 

 doubled conductor. On the question of electric standards. Lord Eay- 

 leigh mentions his own measurements of the Clark cell as 1.435 theo- 

 retical volts, and of the electro-chemical equivalent of silver, 4.025 

 grams per ampere-hour, as closely according with that of Kohlrausch. 

 Another direction of marked progress is in thermo-dynamics. Some 

 confusion has arisen in consequence of applying the first law too rigidly, 

 unmodified by the second. The latter teaches that the real value of 

 heat as a source of mechanical power depends upon the temperature of 

 the body in which it resides. Every change, chemical, thermal, or me- 

 chanical, which takes place or can take place in nature does so at the 

 cost of a certain amount of available energy. If, therefore, we wish to 

 inquire whether or not a proposed transformation can take place, the 

 question to be considered is whether its occurrence would niA'olve dissi 

 pation of energy. If not, the transformation is absolutely excluded 

 • S. Mis. 33 28 ^'^'^ 



