322 Mr. H. A. Rowland on the Magnetic Permeability 



ing upon magnetic induction. Thus if we place an electromag- 

 net near a compass-needle, the Germans would say that the 

 action was due in part to two causes — the attraction of the coil, 

 and the magnetism induced in the iron by the coil. Those who 

 hold Faraday's theory, on the other hand, would consider the 

 substance in the helix as merely " conducting " the lines of force, 

 so that no action would be exerted directly on the compass- 

 needle by the coil, but the latter would only affect it in virtue 

 of the lines of force passing along its interior, and so there 

 could be no attraction in a perfectly vacant space. 



According to the first theory, the magnetization of the iron 

 is represented by the excess of the action of the electromagnet 

 over that of the coil alone; while by the second, when the coil 

 is very close around the iron, the whole action is due to the 

 magnetization of the iron. The natural unit of magnetism to 

 be used in the first theory is that quantity which will repel an 

 equal quantity at a unit's distance with a unit of force ; on the 

 second it is the number of lines of force which pass through a 

 unit of surface when that surface is placed in a unit field per- 

 pendicular to the lines of force. The first unit is 4m times the 

 second. Now when a magnetic force of intensity S$* acts upon 

 a magnetic substance, we shall have 23 = »£) + 47r3, in which 23 

 is the magnetization of the substance according to Faraday's 

 theory, and is what I formerly called the magnetic field, but 

 which I shall hereafter call, after Professor Maxwell, the mag- 

 netic induction. 3 is the intensity of magnetization according 

 to the German theory, expressed in terms of the magnetic mo- 

 ment of the unit of volume. Now, when the substance is in the 

 shape of an infinitely long rod placed in a magnetic field parallel 



to the lines of force, the ratio -£•=/>& is called the magnetic per- 



meability of the substance, and the ratio ~E s =fc is Neumann's 



coefficient of magnetization by induction. Now experiment 

 shows that for large values of «£> the values of both p, and k de- 

 crease, so that we may expect either 3 or both 23 and 3 to attain 

 a maximum value. 



In my former paper I assumed that 23 as well as 3 attain a 

 maximum ; but on further considering the subject I see that we 



* I shall hereafter in all my papers use the notation as given in Pro- 

 fessor Maxwell's ' Treatise on Electricity and Magnetism; ' for comparison 

 with my former paper I give the following : — 



23 in this paper =Q in former one. 

 § „ =4ttM 



3 „ = -^-M „ 



