520 Double Solenoid for Uniform Magnetic Fields. 



In order to produce fields of moderate intensity in ordinary 

 circumstances, a solenoid must have at least several layers. 

 Such a coil may be constructed with precision, as Bestel- 

 meyer * has pointed out, by winding one layer as described 

 above, and with pitch considerably less than twice the 

 diameter of the wire, winding a second layer in the same 

 direction in the depressions between the first, and so on. A 

 serious defect of this arrangement, however, is due to the 

 long conductors necessary to connect the far end of each 

 coil with the near end of the next, which may interfere 

 greatly with the direction and uniformity of the field due to 

 the spiral windings. 



All the advantages of precise winding maybe obtained, 

 and at the same time the (small) effect of spirality on the 

 uniformity of the field may be largely eliminated and the 

 trouble due to connectors and leads avoided, as follows. 

 Two solenoids are constructed with the same pitch and 

 number of layers, and with practically the same length, but 

 with somewhat different diameters, so that one may be 

 placed inside the other. One coil is wound in left-handed 

 spirals, the other in right-handed spirals, and the two are 

 mounted coaxially and concentrically. The construction 

 necessitates that the successive layers in each solenoid start 

 from points 180° apart. When each coil has two or more 

 layers it is probably best to wind all layers of one coil with the 

 same integral number of turns, and all layers of the other coil 

 with this same number of turns plus or minus one half-turn. 



It is then clearly possible to join successive layers of the 

 two solenoids systematically in series by very short con- 

 nectors at the ends, in such a way that the current goes 

 alternately up a layer of one solenoid and down a layer of 

 the other, and that the small effects of each pair of short 

 connectors are practically cancelled near the centre of the 

 field. Difficulties are encountered in making the windings 

 precise close to the ends, but slight irregularities there are 

 of little consequence. 



The same general method of construction, of course, applies 

 to solenoids of all dimensions. In a large double solenoid 

 constructed in this laboratory the coils are wound on tubes 

 of bakelite-dilecto, which is known to have excellent mag- 

 netic and mechanical properties, and to insulate so well that 

 any kind of current may be used. It can be worked readily 

 with a diamond tool. 



Department of Terrestrial Magnetism, 

 Carnegie Institution of Washington. 



* Phys. Zeit. 1911, p. 1107. 



