688 POPULAR SCIENCE MONTHLY. 



we see at A. The filings arrange themselves in the direction of 

 the magnetic force, which we see to be in concentric circles around 

 the wire. According to theory, this magnetic force extends to an 

 indefinite distance. Near the wire the force is very strong, but 

 grows weaker, losing strength with distance until it finally be- 

 comes imperceptible. Before we connected the wire to our battery 

 this force did not exist. Where did it come into existence first 

 near the wire or far from the wire or did it come suddenly 

 everywhere at once ? The late Dr. Hertz performed some won- 

 derful experiments in this connection, in which he showed that 

 the magnetic force comes into existence first near the wire and 

 then makes its appearance a little farther off, and so on till all 

 the surrounding space is filled with the force. Dr. Hertz's experi- 

 ments seem to indicate, moreover, that the rate at which this 

 magnetic force travels out from the wire is perfectly definite ; in 

 a word, that it travels with the velocity of light. You can picture 

 this to yourself by imagining the wire suddenly to emit light 

 just as we connect it to the battery ; then the light and the mag- 

 netic force will both reach any point at which you may place 

 your eye at exactly the same time. 



This is the theory, and a very interesting one it is, but it does 

 not stop here, for not only does this magnetic force travel with 

 the velocity of light, but it has been proved by experiment that 

 it can be reflected, refracted, and brought to a focus. 



Many observers are now engaged in reproducing and extend- 

 ing Dr. Hertz's experiments, and many brilliant results are to be 

 expected. 



Now, to see why the velocity determined between Cambridge 

 and St. Louis was not the velocity of electricity, we must go back 

 to some fundamental principles which at first sight seem to have 

 no connection with the question. 



Just as a current of electricity produces magnetic force around 

 the wire carrying the current, so does magnetic force around a 

 wire produce a current of electricity, no matter how the magnetic 

 force may be produced ; but, whereas the current produces a mag- 

 netic force that lasts as long as the current flows, the magnetic 

 force produces a current only while the force is growing, so to 

 speak while it is being made. If, now, we have a wire, a, so ar- 

 ranged that a current of electricity may be sent through it from 

 a battery by pressing a key, and another wire, 6, parallel to a, 

 connected with an instrument for detecting a current of elec- 

 tricity, when we press the key we shall get magnetic force around 

 a, extending as it grows to h. While this magnetic force is grow- 

 ing, we find there is a current through h in the opposite direction 

 to the current through a. Now let us move h up closer to a. We 

 get, of course, the same effect, only the current in h is stronger 



