216 Dr. S. P. Thompson and Mr. Miles Walker on 



The first position in the above table is the most important, 

 as the coil is placed in a part of the field that experiences the 

 greatest change by the presence or absence of the iron plate, 

 the difference being that between 236 and 129. If the search- 

 coil could have been placed so that its centre was at the very- 

 end of the solenoid, the throw ought to be just one half of 

 what it would be with the coil continued to double its length. 

 This was found to be so when the solenoid was placed on the 

 other solenoid, but could not be tried with the iron as part of 

 the search-coil protruded. 



The experiment was then made of lifting the solenoid step 

 by step a little distance from the iron plate, when it was 

 found that the presence of the iron plate had the effect of 

 producing an image of the coil receding an equal amount 

 behind the plate. 



Fig. 3. 



COIL 



search coil 



iron jplate 



\mmmmmmmmmmmmmmmmmmmmmmmmm&. 





IMAGE 



A larger search-coil was placed in different positions out- 

 side the solenoid ; and in all positions on which any readable 

 throw of the needle could be obtained, it was found that the 

 iron plate gave an effect almost exactly the same as an equal 

 solenoid in the position of its image. 



In order to try the effect of tilting the solenoid, it was 

 mounted on a strip of wood hinged to another, the angle 



Fig. 4. 



between them being fixed by a clamp (fig. 4.) This was then 

 placed alternately on the iron plate and against a similar 



