TRANSACTIONS OF SECTION A. 



657 



11. On the Physics of the Voltaic Arc. 

 Bij Professor Silvanus P. Thompson, F.B.S. 



Temperature Variation of the Magnetic 

 Bij Edwin H. Barton, B.Sc, and W. 



12. Preliminary Note on- the 

 Permeability of Magnetite. 

 Williams, B.Sc. 



1. After being' eng'aged for some time upon the ' Magnetic Survey of the 

 United Kingdom,' Professor Riicker put forward his ' Theory of Terresti'ial Mag- 

 netism ' C Proc. Royal Soc.,' vol. xlviii. June 19, ISiJO, ' Relation between 

 Magnetic Permeability of Rocks and Regional Disturbances '). 



In connection with this theory it was desirable to ascertain the permeabilities of 

 various rocks both at ordinary and high temperatures. 



The investigation referred to in the present preliminary note is an instalment in 

 this direction. 



2. Professor Riicker proposed this imdertaking, and indicated the plan to be 

 pursued. It was then carried out under his supervision at the Royal College of 

 Science, London, as follows: — 



W. Williams worked out the theory of the method, calculated what coils would 

 be suitable, and wound them accordiugl)^ 



E. H. Barton arranged the apparatus, eliminated disturbances, took the 

 observations, and plotted the results. 



3. For measuring the susceptibility of the magnetite the ballistic method was 

 adopted. 



Since the specimen was to be exposed to high temperatures, the coil to receive 

 it was insulated with asbestos paper, and wound upon a porcelain tube. The 

 secondary coil was further insulated from the primary by two sheets of mica to 

 obviate the probability of leakage between the two. 



The temperature of the magnetite, when heated, was estimated from the con- 

 sequent E.M.F. of a thermo-electric couple of platinum and osmium-iridium. 



Fig. 1. 



M )r, Specimen of magnetite ; p and s, primary and secondary coils, heated by gas stove (not 

 shown in figure) ; p' and s', compensating coils ; b, battery ; a, ammeter ; r, adjustable 

 resistance ; k, a ' Pohl's ' commutator ; c, compensating coil ; g, highly sensitive low- 

 resistance galvanometer. 



One of the junctions was round the specimen of magnetite, and the other in a 

 beaker of water. This couple was first calibrated by placing one junction in 



1892. uu 



