Mirrors of Magnetism. 



219 



2 metres long and 1*5 centim. mean diameter, was "uniformly 

 wound with twelve turns per centimetre of a suitable wire. 

 When a current of 15 amperes was passed through this a 

 pole of a strength of about 40 units (G.G.S.) was obtained, 

 and this might be regarded as an isolated pole at distances 

 not greater than 30 centim. from it. It was found that 

 bringing an iron plate within a few centimetres of this pole 

 produced very nearly the same effect upon the field as would 

 have been produced' by bringing up a pole of opposite sign to 

 a point at double the distance. 



Fig. 9. 



Fig. 10. 



p~P3 



^SSiSSftSSSSSSSS^^SS^^ 



The field close to the iron plate ought to have been exactly 

 twice as strong as if no iron plate were there. This was found 

 to be so within the limits of error of observation (about 3 per 

 cent.) when the pole was within 6 centim. of the iron plate. 

 As the distance was increased the throw of the needle became 

 too small to be accurately read, and the field being extremely 

 weak the susceptibility of the iron would be very low. 



It was this weakness of the field (for one could not consider 

 the pole as a point except at distances exceeding 10 centim.) 

 that spoilt some experiments on kaleidoscope effects. One 

 can conceive that if the laws of reflexion hold good between 

 a magnetic pole and a sheet of iron, it ought to be possible, by 

 means of several sheets of iron, to build up a magnetic kaleido- 

 scope by which the effect of a great number of magnetic poles 

 could be produced from a single pole. The idea of a mag- 

 netic kaleidoscope was suggested by Lord Kelvin many years 

 ago, but it is doubtful whether it has ever been successfully 

 carried out. If, for instance, two plates of susceptible 



