506 Prof. Riicker. On the Magnetic Permeability of 



In selecting a method, it was important to be able to deal with 

 small fragments of rock, and also to avoid the necessity of having to 

 shape them into definite forms. The magnetic properties of a dis- 

 trict can only be ascertained by the examination of a large number of 

 specimens, and this would be practically impossible unless the above 

 conditions were fulfilled. Great accuracy, though desirable, is not 

 equally important. Different specimens of the same rock differ so 

 widely in their magnetic qualities that a comparatively rough 

 measurement is sufficient. It will, however, be seen in the sequel 

 that the observations made are in satisfactory agreement. 



To meet these requirements the following scheme was devised. A 

 series of standard magnetic fluids were made by suspending magnetic 

 oxide of iron in various proportions of glycerine. The susceptibilities 

 of these mixtures were determined absolutely by the apparatus 

 described below, and specimens of the rocks were compared with 

 them by means of Professor Hughes' induction balance. For this 

 purpose, equal volumes of a mixture were placed in two similar test- 

 tubes, which were inserted in the cups of the balance, and silence was 

 obtained by means of a compensator. The rock to be tested was 



FIG. 1. 



now immersed in one of the mixtures, and an equal volume of liquid 

 having been abstracted, the zero was redetermined. Two mixtures 

 were thus found, to the susceptibilities of which that of the rock 

 under experiment was intermediate. The compensator used, though 

 identical in principle, differed in form from that employed by Professor 

 Hughes. The primary current passed through two solenoids wound 

 in opposite directions about the two ends of a tube. Over these 

 another larger tube could be moved in either direction, and round it 

 another solenoid was wound which formed part of the secondary cir- 

 cuit. The position of this secondary solenoid was read off on a milli- 

 metre scale attached to the exterior of the inner tube. The two 

 primary coils tended to produce induced currents in opposite direc- 

 tions, and thus, by moving the secondary coil in one direction or the 

 other, silence could be obtained. 



If iCj and x% are the distances (measured in opposite directions) 

 through which the compensator had to be moved to produce silence 

 when the specimen was introduced into the first and second liquid 



