( 567 ) 



XV. — The Temperature Variation of the- Magnetic Permeability of Magnetite. By 

 Edwin H. Barton, D.Sc. (LoncL), F.R.S.E., A.I.E.E., Senior Lecturer and 

 Demonstrator in Physics at University College, Nottingham. (Plates I. -III.) 



(Read February 3rd, 1896.) 

 CONTENTS. 



SECTION . PAGE I SECTION PAGE 



I. Introduction, 567 



II. Apparatus, 568 



III. Disturbances and their Elimination, . . 568 



IV. Method of Experimenting and Results, . 571 



V. Discussion of Apparent and True Sus- 



ceptibilities, 572 



VI. Absolute Values of True Susceptibility, . 573 



I. Introduction. 



1. Relation to Magnetic Survey. — After being engaged for some time, in conjunction 

 with Professor Thorpe, upon the Magnetic Survey of the United Kingdom, Professor A. 

 W. Rucker, F.R.S., put forward his theory of Terrestrial Magnetism.* 



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

 various rocks, both at ordinary and at high temperatures. 



2. The investigation which forms the subject of the present paper is an instalment 

 in this direction. Professor Rucker proposed this undertaking and indicated the plan 

 to be followed. It was then carried out under his supervision at the Royal College of 

 Science, London. 



3. Method. — The Ballistic Method of measuring the susceptibility was adopted. 

 The specimen of magnetite is placed in the magnetic field produced by a long primary 

 coil in the form of a helix. Round the middle of this is a short secondary coil. Upon 

 the reversal of the current in the primary coil, a transient induced current is sent by 

 the secondary coil through a galvanometer in series with it. 



The true effect sought is that due to the introduction of the specimen of magnetite. 

 This depends upon its magnetic susceptibility and its dimensions. The effect at first 

 obtained is one in which the desired result is complicated, and, it may be, masked by 

 other causes. From these it has to be disentangled, as will be explained later. 



4. Arrangements for High Temperatures. — Since the specimen of magnetite 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. 



* " Relation between Magnetic Permeability of Rocks and Regional Disturbances," Rucker, Proc. Roy. Soc, vol. 

 xlviii., June 19, 1890. 



VOL. XXXVIII. PART III. (NO. 15). - 4 I 



