322 Intelligence and Miscellaneous Articles. 



described from Snead, near Bishop's Castle, where fragments of basic 

 glass are imbedded in a tuff of Ordovician age. 



In conclusion, the well-known variolite of the Durance was cited 

 as a rock of basic character, comparable, in its perlitic and spheru- 

 litic structures with the acid " pyromerides," both types having alike 

 suffered from secondary devitrifiction. 



4. " Appendix to Mr. A. T. Metcalfe's paper ' On Further Dis- 

 coveries of Vertebrate Eemains in the Triassic Strata of the South 

 Coast of Devonshire, between Budleigh Salterton and Sidmouth.' " 

 By H. J. Carter, Esq., F.K.S. 



XLIII. Intelligence and Miscellaneous Articles. 



ON THE PRODUCTION OF INTENSE MAGNETIC FIELDS. 

 BY PEOF. STEFAN. 



THE maximum magnetizing force which cylindrical iron cores of 

 an electromagnet can exert between plane pole-pieces standing 

 opposite each other is 4ttju, in which /j. is the greatest magnetic 

 moment which the unit volume of iron can assume. If we take 

 ju = 1700 absolute units, 47r/x = 213600. This value is the more 

 nearly attained, the smaller the distance of the pole-pieces. 



The action which the iron cores exert in the centre of the field 

 can be increased without limitation of its length, if the terminal 

 faces of the pole-pieces are made of another shape. This increase 

 attains the greatest value if the ends of the cores have the shape of 

 truncated cones, so that the generating lines of the two conical 

 surfaces pass through the centre of the field, and form with its axis 

 an angle whose tangent is *J2, that is an angle of 54° 44'. 



The magnitude of the force is then expressed by the formula 



H = 47r,u (0-289 + 0-886 log — ), 



where d is the diameter of the iron core, a the length of the field. 

 The formula gives an increase of H to any large values. In practice 

 this is unimportant, for with the slow increase of the logarithms 

 compared with that of the numbers, conditions of construction 

 result which cannot be carried out. If H = 8t/x, d must be taken 

 = 85a. 



For certain optical investigations it is necessary to perforate the 

 iron cores. For such cores with plane pole-pieces the principle no 

 longer holds that the magnetic field is the more intense the nearer 

 the pole-surfaces. The magnetic force in the centre of the field 



