156 ELEMENTARY LESSONS ON [CHAP. in. 



191. Magnetic field due to Current. Arago 

 found that if a current be passed through a piece of copper 

 wire it becomes capable of attracting iron filings to it 

 so long as the current flows. These filings set them- 

 selves at right angles to the wire, and cling around it, 

 but drop off when the circuit is broken. There is, then, 

 a magnetic " field," around the wire which carries the 

 current ; and it is important to know how the lines of 

 force are distributed in this field. 



Let the central spot in Fig. 8 7 represent an imaginary 

 cross-section of the wire, an& *ct us suppose the current 

 to be flowing in through the paper at that point. Then 

 by Ampere's rule a magnet needle placed below will tend 

 to set itself in the position shown, with its N. pole 

 pointing to the left. 1 The current will urge a needle 

 above the wire into the reverse position. A needle on 

 the right of the current will set itself at right angles to 

 the current (i.e. in the plane of the paper), and with its 



N. pole pointing down, 

 while the N. pole of a 

 needle on the left would 

 be urged up. In fact the 

 tendency would be to urge 

 the N. pole round the 



Fig. 83. Fig. 84. j ..-U 



conductor in the same 



way as the hands of a watch move ; while the S. pole 

 would be urged in the opposite cyclic direction to that of 

 the hands of a watch. If the current is reversed, and is 

 regarded as flowing towards the reader, i.e. coming up 

 out of the plane of the paper, as in the diagram of Fig. 



1 If the student has- ^ m applying Ampere's rule to this case and 



the others which succeed, he should carefully follow out the following mental 

 operation. Consider the spot marked " in " as a hole in the ground into 

 which the current is flowing, and into which he dives head-foremost. While 

 in the hole he must turn round so as to face each of the magnets in succession, 

 and remember that in each case the N. -seeking pole will be urged to his left. 

 In diagram 84 he must conceive himself as coming up out of the hole in the 

 ground where the current is flowing out. 



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