Double Solenoid for Uniform Magnetic Fields, 519 

 Table II. 



From Crystal measurements. From Viscositj measurements. 



Element. 



Atomic 



diameter. 



Chlorine. 

 Bromine. 

 Iodine . 



, 10 



2'38 j 



2-80 J 



Increment. 



Atomic 

 diameter. 



254 



0-28 



0-42 



2 98 



Increment. 



0-17 

 0-27 



4. Oxygen and Nitrogen. — his well known that the kinetic 

 theory has indicated that the molecules of these two gases 

 are of: approximately the same dimensions. The actual 

 figures are somewhat uncertain, owing to inconsistencies in 

 the values of the viscosities obtained by different observers. 

 Approximate calculation based on the assumptions indicated 

 in the last paragraph indicate that the molecules o£ both 

 nitrogen and oxygen resemble dimensionally two atoms of 

 neon in contact. This agrees with W. L. Bragg's mea- 



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surements, and affords additional justification for his as- 

 sumption that the diameter of the outer electron shell of neon 

 is practically identical with those of the incomplete outer 

 shells of nitrogen and oxygen. 



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LXIII. A Double Solenoid for the Production of Uniform 

 Magnetic Fields. By S. J. BARNETT *. 



THE universal use and great importance of the solenoid 

 in magnetic research have led me to believe that a 

 brief account of a method of construction devised here will 



be useful. So far as my knowledge goes, it is the first 



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description of a precise method of winding a solenoid of 

 more than one layer, and at the same time eliminating the 

 troublesome effects of leads and interconnexions. 



Probably the best way in which a single-layer coil can be 

 constructed with great precision is by winding uniform 

 round wire in a spiral groove accurately cut in a circular 

 cylinder, as first suggested by Viriamu Jones and as exem- 

 plified in the work of the National Physical Laboratory. 

 If the cylinder is conducting and lone, as in the case of a 

 solenoid, the effect of the leads may be practically eliminated, 

 as they often are, by connecting one end of the coil to the 

 cvlinder and using the other end as one terminal ; but such a 

 coil cannot be used satisfactorily with alternating currents. 

 * Communicated by the Author. 



