. A text-book of electrical engineering;. the air, which is representedby the letter H, is therefore also called the magnetising force. Thenumber of lines per square centimetre in the iron is called the magneticinduction and is represented by the letter B. It is to be noticed that 62 Electrical Engineering the induction B includes the Hues which were present before the ironwas introduced as well as the new lines due to its introduction. We account for the greatly increased number of lines in iron by assumingthat its magnetic conductivity or permeability is much greater than that ofair. The rat
. A text-book of electrical engineering;. the air, which is representedby the letter H, is therefore also called the magnetising force. Thenumber of lines per square centimetre in the iron is called the magneticinduction and is represented by the letter B. It is to be noticed that 62 Electrical Engineering the induction B includes the Hues which were present before the ironwas introduced as well as the new lines due to its introduction. We account for the greatly increased number of lines in iron by assumingthat its magnetic conductivity or permeability is much greater than that ofair. The ratio of this permeability to that of the air is represented by theGreek letter /a. The permeability jn gives, therefore, the magneticconductivity of the iron compared with air; other things being equal,the number of lines produced in the iron is fx, times the number whichwould have been produced in the air. From this it follows that B = (45), B or ^ = H .(46). For air the permeability is evidently i, and we have B = H. [\-\-\-yNWr--i. Fig. 55 The value of fi depends above all on the quality of the iron. The per-meabihty of annealed armature stampings or of mild cast magnet steelmay sometimes exceed 3000. The permeability of a given piece of ironor steel varies moreover with the degree to which it is magnetised. The experimental determination of the permeabiUty may be carried outin the way indicated in Fig. 55. The rod of iron to be tested is made in twoparts, and passes through holes a and a^ in a massive iron yoke and alsothrough the magnetising coils S and S^, and the small secondary coil current I is passed through the coils S and S^, and from a knowledge ofthe number of turns in these coils, together with the length I of the ironrod, the value of H can be found from the equation . _ 0-47r .1 .T H-- I The length I does not include the parts of the rod inside the holes a and a^,as these carry very little of the magnetic flux, which leaves the rod in alldirections as s
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