66 Messrs. Ayrton, Mather, and Sumpner 



Since winding wire inside the surface whose section is 

 given in fig. 4 would oppose the effect of the wire wound 

 outside this surface if the current flowed in the same sense 



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Shape of cavity to be left unwound inside the coil when the length of the 

 magnetic needle is 2. 



throughout the whole coil, it is possible, by causing the 

 current to circulate in opposite directions in the two portions, 

 to wind all the space and to cause all the convolutions to help 

 one another. This we have not yet tried, but in view of the 

 fact that the space which has been left unwound in the gal- 

 vanometers constructed according to Mr. Mudford's suggestion 

 is very near the needle, and therefore very valuable, it seems 

 important to try and utilize it in the way just suggested. In 

 that case the surface we have theoretically determined would 

 be the surface separating the coil into two parts, wound 

 respectively in opposite directions. 



