460 Prof. A. W. Riicker and Mr. W. H. White. 



" On the Determination of the Magnetic Susceptibility of Rocks." 

 By A. W. RUCKER, Sec. R.S., and W. H. WHITE, A.R.C.S. 

 Received July 6, 1898, and published during the vacation. 



In the Report of the Magnetic Survey of the United Kingdom* 

 the question as to how far the local and regional magnetic dis- 

 turbances of the earth's field may he ascribed to the influence of 

 masses of magnetic rocks was discussed by Professors Riicker and 

 Thorpe.f 



In a previous paperj a method for the determination of the 

 magnetic susceptibilities of rocks was given, together with values 

 for the specimens then examined. It was considered desirable to add 

 to this relatively small number of results so as to obtain a broader 

 basis for the development of the theory. Accordingly Sir Archibald 

 Geikie, F.R.S., Director- General of the Geological Survey, has been 

 good enough to supply us with a typical series of specimens of the 

 basic rocks of the country, and, after a complete overhauling of the 

 method originally adopted, we have measured the susceptibility of 

 sixty-eight of these new specimens of forty-five different rocks. The 

 earlier stages of the work were carried out by Mr. F. Fisher. A.R.C.S., 

 who would, no doubt, have taken part in the whole had he not left 

 the college for an appointment elsewhere. 



The principle of this method of determining the susceptibility of 

 small pieces of feebly magnetic materials is to compare the suscepti- 

 bility of the fragment with that of the displaced volume of liquids 

 of known susceptibility in which it is immersed. 



In more detail, the method is as follows : 



(a) To measure the susceptibility of the magnetic liquids a tube 

 containing the liquid is introduced into one of two equal solenoids, 

 the effects of which on a small magnetometer destroy each other. In 

 these experiments a field of seven absolute units WAS employed. The 

 ratio of the deflection caused by the tube of liquid to that produced 

 by a measuring solenoid traversed by a small current, multiplied by 

 the computed constant of the instrument, gives the absolute sus- 

 ceptibility of the liquid. 



The apparatus was figured and described on pp. 506 509, ' Roy. 

 Soc. Proc.,' vol. 48, 1890. 



(6) The differences of susceptibility of rocks and liquids are 

 measured on the calibrated sonometer scale of a Hughes' induction 

 balance. Two tubes, filled with a liquid to the sufficient depth of 

 6 cm., are balanced, and the distance through which the compensator 



* ' Phil. Trans.,' vol. 188, 1896- 



f Vide pp. 637 647 and map 14. 



T ' Boy. Soc. Proc.,' vol. 48, 1890, p. 505. 



