312 CARNEGIE INSTITUTION OF WASHINGTON. 



Particular attention is called to the steep pjradient between the lines of equal 

 annual change in the South Atlantic Ocean, especially in the vicinity of 

 Cape of Good Hope. The variation with time in the annual change of the 

 declination for this locality has been very large, as evidenced by the results 

 of the shore observations made at Cape Town. The annual change was 

 3' E in 1911, whereas in 1920 it was 11' E. 



The last four columns show the corresponding annual changes for declination 

 as given on the latest magnetic charts: British Admiralty for 1917 and U. S. 

 Hydrographic Office for 1920, together with the corrections to be applied to 

 these chart values to obtain the Carnegie values. The second and third values 

 from the bottom of the table are in the Indian Ocean, but are included here to 

 show the distribution in the vicinity of Cape of Good Hope. The values 

 are arranged according to latitude, but where several intersections occur 

 near the same locality, the values for these intersections are arranged ac- 

 cording to the dates in order to show up any progressive change or variation 

 with years. The results are subject to slight changes when the final reductions 

 are made at the end of the present cruise of the Carnegie. 



The annual changes for declination and inclination are referred to the 

 north-seeking end of the magnetic needle. Thus 6' W signifies that the north- 

 seeking end of the compass moved to the west at the average annual rate of 

 6' during the period shown in the tliird column of the table; 1' N means that 

 the north-seeking end of the dip-needle moved downward at the average 

 annual rate of 1' during the period in the third column; horizontal intensity 

 is always considered positive, and the signs, plus and minus, have their usual 

 significance. 



A double solenoid for the production of uniform magnetic fields. S. J. Barnett. Philos. 

 Mag., vol. 40, pp. 519-520 (1920). 



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 cyHnder, as first suggested by Viriamu Jones and as exempli- 

 fied in the work of the National Physical Laboratory. If the cylinder is 

 conducting, and long, 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 cylinder and using the other end as one terminal, but such a coil 

 can not be used satisfactorily with alternating currents. 



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 Bestelmeyer^ 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 arrangment, 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 may be 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 jiitch 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 



iPhys. Zeit. 1911, p. 1107. 



