152 PHYSICAL SCIENCE 



hood when those bodies are positively electrified, 

 and to repel them if their charges be negative. 

 Rejecting the idea of action at a distance, Faraday 

 regarded these electric forces as transmitted by 

 stresses and strains in the dielectric or Insulatlno- 

 medium, and represented the state of that medium 

 by a series of lines, drawn everywhere so as to 

 lie in the direction of the force on a positively 

 electrified particle. 



The distribution of these electric lines of force 

 can be investigated theoretically, the laws of force 

 being known, but it is not easy to illustrate them 

 experimentally. On the other hand, the corre- 

 sponding magnetic lines can be rendered visible 

 and mapped out by a familiar experiment, which, 

 indeed, first suggested to Faraday his conception 

 of lines or tubes of force. If the poles of a 

 horse-shoe magnet be placed beneath a sheet of 

 cardboard, over which iron filings are sprinkled, 

 a picture of the magnetic lines of force is formed 

 by the filings (Fig. 31). Under the influence of 

 the magnetic field, each filing becomes a little 

 magnet, and attracts others, forming chains of 

 filings which lie everywhere in the direction of 

 the magnetic force. Where the force is strong, 

 the filings cluster thickly ; where the force is 

 weak, few filings are to be seen. Thus a 

 complete representation of the lines of magnetic 

 force is obtained. 



The laws of force are similar for electric 

 charges and for magnetic poles, and the lines of 

 force will possess the same form. Thus the 

 filings in Fig. 31 represent also the direction and 

 distribution of the electric lines or tubes of force 

 in the neighbourhood of two electric charges of 



