226 Prof. J. Perry on a Formula for Calculating 

 for practical purposes we may take, when c=l centim., 



L a 2 



or / = 



4-rm 2 ~ -2317a + -39 + -44&' 



Hence generally, from the law as to similar coils, I is 



(2) 





•2317 -^+-39 + -U±' ° r '2317a +-39c + -446- 

 So that generally 



L in centimetres = ™ u a . . (3) 



•2317« + , oyc + *44o v y 



I give it in this form to enable its accuracy to be tested. And 

 it is obvious that q of (1) is a constant, whereas p is a linear 



function of - including a constant. 



t . n a 



L in centimetres = ^. tl . 77771 tttt^t. . (4) 



•01844 a + -031 c + *035 b v y 



Or taking the ohm as 10 9 centimetres per second, 



n 2 a 2 — 10 7 

 L in secohms = Q . — -^1 — , o K 7 • • . (5) 

 1*844 a + 3-1 c + 3*5 6 v y 



If a is the cross sectional area pertaining to one winding, 

 whether made up partly of insulation or not, 



be 

 n= — ; 



OL 



and if R is the resistance of the coil in ohms, as R = £— , 



7 or 



L V-?-10Vp 



(6) 



R •2317a + -39c + '445' * * ' 



where V is the volume of the coil in cubic centimetres, p = 

 resistance in ohms of a copper wire 1 centim. long, of smaller 

 section than 1 centim., just as much smaller as the actual wire 

 is than the covered wire. 



Let p = specific resistance of copper in ohms. 



Taking it that if there is no insulation, p=p =l'62 x 10" 6 . 

 Let Wi — weight of copper in the coil (disregarding the 



weight of the insulating material, this is the 



weight of coil), 

 "W 2 = weight of volume of the coil of copper, 



W, = = P 



Wj section of copper p 



