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II. — Circular Magnetisations accompanying Axial and Sectional Currents along Iron 

 Tubes. By Professor Cargill G. Knott, D.Sc, F.K.S.E. (With Plate.) 



(Read 18th January 1892.) 



The experiments now to be described have for their object the investigation of the 

 magnetic induction in an iron conductor under the influence of a current passing through 

 it. The method of experiment was briefly in this wise. An iron tube was magnetised 

 circularly by a current passed from end to end along its entire length ; and the induction 

 so produced in the iron was measured in terms of the current induced in a coil of wire 

 wound longitudinally round the walls of the tube. 



In the experiments four tubes were used, all of the same length and nearly the same 

 external diameter. The internal diameters of one pair were approximately double those 

 of the other. The various dimensions are given accurately in the following table, the 

 tubes being distinguished as A, B, a, b. Each diameter measurement is the mean of 

 eight measurements taken across different diameters. 



Length of Tubes = 3 4 "8 cm. 



T , Diameters in cm. 



Internal. External. 



A, .... 1-031 ±-012 3-022 ±-003 



B, .... 1-050 ±-010 3-027 ±-003 

 a, .... 2-036 ±-004 3"022 ± "003 

 6, .... 2-052 =f005 3-021 ±'002 



The tubes were all of wrought iron, turned on the lathe and bored. The a tube was 

 made at a much later date than the other three. The results given below show that it 

 was made out of quite a different specimen of iron. 



To measure the circular magnetisation, four turns of insulated copper wire were 

 coiled longitudinally round the walls of the tubes A, B, and b ; and sixteen round a. 



Each tube could be magnetised circularly in two ways : — 



(1) By an axial current passing along a copper wire led through the axis. 



(2) By a sectional current passing longitudinally through the substance or 



wall of the tube. 



It was by a direct comparison of the inductive effects of these two methods of apply- 

 ing the magnetising force that the action of the latter was studied. To this end two of 

 the tubes were taken and arranged so that the same current could be passed axially or 

 sectionally along them. The induced currents produced in the coils were then balanced 

 on a galvanometer exactly as in the familiar method for comparing mutual inductances 



VOL. XXXVII. PART I. (NO. 2). C 



