148 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



3. That tlie thickness of the magnetic helix should be equal 

 to the diameter of the magnetic bars that it incloses. 



4. That the length of the magnetic helix should be equal 

 to the diameter of the bars multiplied by eleven, and practi- 

 cally by twelve, because of the thickness of the insulating 



- wire. B, 1873, 348. 



THE EOTATIOX OF THE PLANE OF POLAEIZATION UNDER 



MAGNETIC INFLUENCES. 



Since Faraday made the discovery that the plane of a 

 polarized ray of light can be changed by the influence of 

 magnetism, physicists have frequently made this the sub- 

 ject of their investigations. It has been established that the 

 ratio of the change to the intensity of the magnetic current 

 depends upon the nature of the substance, upon the degree 

 of concentration of the solutions (in case such are experi- 

 mented upon), and upon the temperature of the body under 

 observation. Quite recently, Bichat has laid before the scien- 

 tific world a very extended experimental investigation uj^on 

 this whole subject, in which, besides confirming many previ- 

 ous results, he establishes some new and important facts, 

 which may be summed up as follows: 



Transparent bodies, whether they are in a solid condition 

 or dissolved in liquids, are still subject to the magnetic influ- 

 ence. The power of this magnetic influence diminishes as 

 the temperature rises, and it entirely disappears when the 

 body is converted into vapor. For instance, notwithstanding 

 he applied the most powerful means, Bichat was unable to 

 find the least trace of a change in the plane of polarization 

 when 'the ray of light passed through the vapor of the chlo- 

 ride of tin. 19 (7, 1873, 281. 



THE TEMPER OF STEEL AND ITS MAGNETIC POWER. 



Jamin has made an excellent study of the magnetic power 

 of various kinds of steel, as well as of that which has been 

 tempered to a greater or less degree. He measures the pow- 

 er of his magnet by the weight that it will support, using, 

 of course, magnets of the same size, and exposing the same 

 surface to their ai'matures. He finds that soft iron does not 

 retain its magnetism so long as tempered iron, while, on 

 the. other hand, those kinds of steel which liave but little 



