GENERAL ACCOUNT OF MAGNETIC ACTIONS 179 



poles where there are no lines, and a compass needle at that point 

 shows no directive force. 



If two bar magnets are placed in line with unlike poles in con- 

 tact, then they act very nearly as one magnet with poles near the 

 two distant ends, and very little polarity is shown at the junction. 

 But if they are separated by a small gap a strong field is exhibited 

 in the gap. 



When the course of the lines of force in a field has been traced, 

 as in any of the above figures, a compass needle shows us by its 



\ 



"K 



///// 

 '" / .\^i///; 



FIG. 125. 



frequency of vibration that, where the lines crowd together, there 

 the action is stronger ; where the lines open out from each other, 

 there the action is weaker. We shall see later that the lines may 

 be drawn according to a plan such that the number passing 

 through a unit area, held so that they pass through it perpen- 

 dicularly, serves as a measure of the action. 



We regard the lines of force as symbolising some change which 

 has taken place in the field, some action which is going on there, 

 an action which is indicated by the setting of a needle and by the 

 magnitude of the pulls on its poles. Now we have seen that we 

 always have the two poles on the same bar and that the lines of 

 force leaving one end of the bar in general bend round and enter 

 the other, and above all we have seen that if we break a magnet 

 in two and slightly separate the broken ends, or if, as equivalent 

 to this, we slightly separate two magnets as in Fig. 124, then 

 there is a strong field between the two ends. These facts 

 lead to the irresistible conclusion that the lines of force pass 

 through the magnet, that the inside of the magnet is as it were 

 the origin of the action, and that the polarity over its ends is 

 merely the manifestation at the surface of what is going on within ; 

 or, putting the same idea in another form, the polarity is due to the 

 passage of the lines offeree from the steel into the air. The lines of 

 force * are to be regarded as continuous closed curves issuing from 



* In a later chapter we shall see that it is convenient to introduce a term to 

 denote the alteration due to magnetic action taking place in the field, and the 

 term used is "induction." The lines which are continuous, and which form 

 closed curves, are really lines of induction. But in attempting to get a general 

 idea of the subject it is sufficient to use the idea of the lines of force which are 

 exhibited so directly by experiment. 



