68 



SCIENTIFIC THOUGHT. 



45. 



" Lines < 

 force." 



hood of the poles of magnets ; l inquired into the nature 

 and condition of the region afterwards termed the " field " 

 which surrounded magnetised and electrified bodies ; 

 invented the term " electrotonic state" and " dielectric" 2 to 

 describe the part which the surrounding medium played 

 in the so-called actions at a distance ; and conceived it to 

 be in a state of tension, which he further described by 

 filling it with so-called " lines of force." The region or 

 " field " 3 of magnetic and electric action, filled with 

 these curved lines of force, possessing definite direc- 

 tion and frequency, gave him a clear mental repre- 

 sentation of the direction and intensity of magnetic and 

 electric forces at any point in space in the neighbour- 

 hood of magnets or of electric currents. For Faraday, 

 the lines of force in the magnetic field, from being 

 originally merely a convenient geometrical device, 4 ac- 



1 ' ' By magnetic curves I mean 

 the lines of magnetic forces, how- 

 ever modified by the juxtaposition 

 of poles, which would be depicted 

 by iron filings, or those to which 

 a very small magnetic needle would 

 form a tangent" (Faraday, 'Ex- 

 perimental Researches on Elec- 

 tricity,' 1st series, November 1831, 

 No. 114 note). " When an electrical 

 current is passed throughawire, that 

 wire is surrounded at every part by 

 magnetic curves, diminishing in 

 intensity according to their distance 

 from the wire. . . . These curves, 

 although different in form, are per- 

 fectly analogous to those existing 

 between two contrary magnetic 

 poles opposed to each other" (ibid., 

 2nd series, January 1832, No. 232). 



2 The term "electrotonic state" 

 was introduced in 1831 to describe 

 the condition of matter in the 

 neighbourhood of electric bodies. 

 " It is probable that what will affect 



a conductor will affect an insulator 

 also, producing, perhaps, what may 

 deserve the term of the electro- 

 tonic state" (ibid., No. 1661, 

 1838), " the intervening particles 

 assuming for the time more or less 

 of a peculiar condition, which 

 (though with a very imperfect idea) 

 I have several times expressed by 

 the term electrotonic state" (ibid., 

 No. 1729). "I use the word 'di- 

 electric ' to express that substance 

 through or across which the electric 

 forces are acting " (December 1838, 

 ibid., No. 1168, note). 



3 The term "magnetic field" 

 seems to have been used for the 

 first time in the year 1845 (see 

 ' Exp. Res.,' No. 2252, vol. iii. p. 30). 



4 November 1837 : " I use the 

 term line of inductive force merely 

 as a temporary conventional mode 

 of expressing the direction of the 

 power in cases of induction. . . . 

 The power, instead of being like 



