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 starting or stopping, before it has reached its steady motion or while it is coming to rest, the electric and magnetic forces are undergoing readjustment 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 experiment: to these Maxwell applied the powerful resources of mathematical analysis and reared the splendid structure of the electro-magnetic theory. 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, hypothesis, 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 electricity 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