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XX. Mirrors of Magnetism. 

 By Silvanus P. Thompson, F.R.S., and Miles Walker*. 



OUR knowledge of Electric Images is based almost entirely 

 upon mathematical deduction. It would be difficult to 

 satisfactorily demonstrate their properties by experiment. 

 We are content to take the elegant geometrical proofs that 

 Lord Kelvin has given us, and corroborate these physically at 

 points where the conclusions are within reach of experiment. 

 The subject, therefore, is one of purely theoretical interest, 

 but so beautifully does it elucidate difficult problems in Elec- 

 trostatics that it remains the continual delight of text-book 

 writers. 



Magnetic Images, on the other hand (apart from one or 

 two investigations by the same great master), are entirely 

 neglected, and yet these are much more likely to be practi- 

 cally utilized in the near future than their electrical analogues. 

 It may be in questions of dynamo and instrument shielding, 

 or it may even be in the solution of that commercial problem, 

 the testing of the magnetic properties of iron en masse. 



The existence of magnetic images is suggested by con- 

 siderations altogether apart from the mathematical. 



If we take a solenoid of wire through which a current is 

 flowing and place its end against a large plate of iron, we 

 eliminate the effect of that end upon the distribution of the 

 field about the coil, and so obtain a field on one side of the 

 plate of a shape the same as if the coil were continued to double 

 its length. If we place the coil with its end against a mirror 

 (fig. 1), we see it reflected in the mirror so that it appears to 

 be double its real length. Thus the iron plate gives us mag- 

 netically an image of the coil analogous to the optical image 

 produced by the mirror, geometrically identical indeed with 

 the optical image, virtual and erect. 



If a large iron plate is placed at each end of the coil the 

 field will be the same as if the coil were infinitely long. We 

 know that two mirrors, facing each other, one at each end of 

 the coil, would give it the appearance of stretching away to 

 infinity in both directions (fig. 2) . » 



Of course these optical effects could only be perfectly 

 obtained if the mirror were a perfect reflector ; and to 

 include all possible points of view it would have to be infi- 

 nitely large. So the magnetic effects could only be perfectly 

 obtained if the plates were perfectly permeable and infinitely 

 large. Observe that we are only considering the effect at a 

 point in front of the mirror and in front of the iron plate. 



* Communicated by the Physical Society : read November 23, 1894. 



