It should be noted that brass is a non-magnetic material. Force at one separation = B2A/2μ0 newton.
Force at both separations = B2A/μ0 newton.
Now F = 12 kg wt = 12 × 9.81 = 117.7 N
∴ 117.7 = B2 × 3 × 10− 4/4π × 10− 7 ; B = 0.7 Wb/m2
If B/H curve is drawn, it will be found that for B = 0.7 Wb/m2 , value of H = 45 AT/m.
Now, length of iron path = 40 cm = 0.4 m.
AT required for iron path = 45 × 0.4 = 18
Value of H in the non-magnetic brass plates
= B/μ0 = 0.7/4π × 10− 7 = 557,042 AT/m
Total thickness of brass plates = 0.5 × 2 = 1 mm
AT required = 557,042 × 1 × 10− 3 = 557,
Total AT needed = 18 + 557 = 575
∴ magnetising current required = 557/800 = 0.72 A