PHYSICS 327 



motion, thus encircling the moving charge. The planes of these circles 

 are perpendicular to the straight path along which the charge is 

 traveling. 



As long as the motion and charge remain uniform there will be no 

 change whatever in this magnetic force except that it keeps abreast of 

 the sphere as do the moving lines of electric force on which it depends. 

 As soon as the motion ceases, the magnetic force disappears and soon 

 all is as it was before the motion began. But while the sphere is start- 

 ing or stopping, before it has reached its steady motion or while it is 

 coming to rest, the electric and magnetic forces are undergoing read- 

 justment and this disturbance spreads outward through the ether with 

 a speed precisely equal to the speed of light. Nor is this a chance 

 agreement, for we now know that light consists of nothing more than 

 very rapidly and periodically changing electro-magnetic forces traveling 

 out through the ether from a particular source of electric disturbance, 

 called a luminous body. The ethereal phenomena we have noted around 

 a moving charge faithfully repeat themselves about a wire carrying an 

 electric current and it was here that Faraday found them. 



To the mental images of Faraday — these lines of force which helped 

 him to grapple with the unseen, to form working hypotheses, to experi- 

 ment : to these Maxwell applied the powerful resources of mathematical 

 analysis and reared the splendid structure of the electro-magnetic the- 

 ory. Now that the work is done we may let fall the scaffolding which 

 Faraday's vivid imagination supplied, but we could not earlier have 

 done without it. Here we have the whole chain, mental image, hy- 

 pothesis, experiment, theory. 



As we now take up what we believe to be the relations of electricity 

 to matter, we come in places upon slippery ground and the bases of our 

 faith rest on recent foundations. 



At the outset we encounter one striking difference between elec- 

 tricity and matter. Every free charge exerts a force upon every other 

 charge in the universe, just as every particle of matter exerts a force 

 on every other particle of matter however distant. But with matter 

 the particles are invariably urged toward each other while electric 

 charges may be either drawn together or forced apart, depending on the 

 kinds of charges. We have both positive and negative electricity, but 

 only one kind of matter. 



Just how these two kinds of electricity are different we know little 

 beyond the invariable law that positive attracts negative and repels 

 positive. In some ways positive and negative electricity resemble right- 

 and left-handed things. If the same number of right- and left-handed 

 turns be given to a screw, one hand will precisely undo the work of the 

 other. If the right and left hands be brought together they fit part 

 for part, but two right gloves are a poor pair. On the contrary, there 



