Recent Advances m Electricity. 21 



and, with rapid alternations, has not time to penetrate to the 

 axis. 



Maxwell developed Faraday's views, not only in the way of 

 giving greater precision to the statement of them, but also by 

 extending the meaning of the phrase, " electric current," to 

 include certain electrical changes in nonconductors, and 

 extending Faraday's laws for currents to these changes. He 

 thus built up a theory of electro-magnetic waves in non 

 conductors, according to which the magnetic force is perpen- 

 dicular to the direction in which the wave advances, and the 

 direction of electrostatic force is perpendicular to both of these. 

 Thus, when there is a regular succession of waves of the 

 simplest kind, there will be, at any given point in the medium, 

 a magnetic force acting along a fixed line, and increasing from 

 zero to a maximum, then diminishing to zero, increasing to a 

 maximum in the opposite direction, and returning again to 

 zero, to go through the same changes again. Similar remarks 

 will apply to the electrostatic force which will also exist at 

 the point acting along a line at right angles to the magnetic 

 force ; while the direction in which the waves advance 

 will be perpendicular to both. Further, Maxwell deduced 

 from his theory that the velocity with which electro- 

 magnetic waves travel in air is the same as a certain velocity 

 which electricians very frequently have to speak of, because 

 they have to use it as a multiplier or a divisor whenever they 

 want to pass from the electrostatic system of units to the 

 electro-magnetic, or vice-versa. This velocity has been 

 measured by electrical experiments in several different ways, 

 and has been found by all experimenters to be about equal to 

 the velocity of light. But if electro-magnetic waves travel 

 with a velocity which is nearly or exactly the same as that of 

 light, may not waves of light themselves be electro-magnetic 

 waves ? Maxwell says they are ; and it is now established 

 by a method of experiment originated by Hertz, that electro- 

 magnetic waves very closely resemble waves of light in 

 their behaviour,' and that they travel with a velocity which 

 is nearly or exactly that of light. 



