Page 292 - Electrical Properties of Materials

274 Magnetic materials

Cobalt steel

Tungsten steel

Iron Iron

Alnico 2
Alnico 5
SmCo
Fig. 11.13
The evolution of the magnetic circuit
of moving-coil meters reflects the
progress in magnet materials
development. Coil size and magnetic
field are equal in all five sketches. o r I n o r I n o r I n

1000
500 12
11
10
200 8 9
(BH) max (kJ m –3 ) 100 6 7

50

20
5
10 4
3
5
2
2
Fig. 11.14 1
1
The achieved optimal value of 1880 1900 1920 1940 1960 1980 2000
(BH) max against time. Year

more complicated and much less useful to an engineer. The most important
activity, the search for better magnetic materials, is empirical anyway, and there
are not many magnetic devices clamouring for quantum theory to solve the
riddle of their operation.
I do think, however, that a brief look into the quantum theory of magnetism
will yield some dividends. It is worth learning, for example, how quantum
numbers come into the picture. We have, after all, come across them when
studying the hydrogen atom, so it is not unreasonable to expect them to be
able to say something about magnetic properties. It is also worth knowing that
there is a very simple experiment showing the quantized nature of magnetic