754 NUMERICAL CONSTANTS. 



Water. 



The properties of water are greatly influenced by traces of 

 foreign substances which cannot be detected by analysis. The 

 specific resistance varies, according to different observers, from 

 0-3 megohm (Foussereau) to 7 megohms (Kohlrausch). Ice has 

 at least 100,000 times the resistance of liquid water at the same 

 temperature. 



DIELECTRICS. 



The specific resistance in ohms may be expressed as a function 

 of the temperature by the formula :* 



Log. R = A 



A - B * C * 



Bohemian Glass . . 2-431 13783 0*0495 7'ii.io~ 6 - 15 to 50 



Ordinary ,, . . 2-539 15-005 0*0527 0-37 - 17 ,, 60 



Crystal ..... 2*933 19*224 0*0880 28-07 45 105 



Porcelain .... 2*933 17734 0*0520 7*21 50 ,, 210 



Phosphorus, solid ..... 11*2103 ' OI 475 ~ 22 '55 10 ,, 42 



liquid .... 6*5035 0*00523 -4-34 25 ,, 100 



Ice ......... 9*6006 0-08797 -127*2 -i ,,-17 



At the temperature zero : 



Specific Resistance. 

 Bohemian Glass ...... 6*07. io 13 or 60*7 millions of megohms. 



Ordinary , ....... I*oi2.io 18 ,, 1012 ,, ,, 



Flint ,, ...... i*675.io 19 ,, 1675.10* ,, ,, 



Porcelain ........ 5*421. io 17 ,, 5421. io 2 



After several Minutes of Electrification^ 



Specific Resistance. Temperature. 

 Mica ....... 0*084. io 9 megohms 2O 



Gutta-percha ..... 0*45 ,, 24 



Shellac ....... 9*0 ,, 28 



Ebonite ...... 28-0 ,, 46 



Paraffine ...... 34*0 ,, 46 



1292. ELECTROMOTIVE FORCES. The volt, taken as unit, is the 

 electromotive force capable of maintaining a current of an ampere 

 in a legal ohm. 



* FOUSSEREAU. Ann. de Chim. et de Phys. [6], Vol. v., p. 317. 1885. 

 f AYRTON and PERRY. Proceedings of the Roy. Soc. t March 2ist, 1878. 



