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Isolating Magnetic PolesBeyond shielding sensitive equipment from the effects of external magnetic fields, there are several devices or ideas that make use of the ability of magnetic shields to manipulate the external field into a new ‘shape’ by redirecting magnetic flux.

A popular example is the idea of shielding one magnetic pole from another or keeping only one pole shielded, thereby effectively ‘separating’ the two poles from each other. Such a creation could be used as a form of magnetic switch, or could even shed light on the Grand Unified Theory.

 

As described previously, it is currently impossible to completely separate two poles as there have been no observed magnetic monopoles beyond the level of quasiparticles. However, research into the extent of attenuation in such cases has been conducted in order to depict how useful this field redirection can be in such an application.

 

Several open cylindrical shields of different dimensions were first used in order to investigate the extent of shielding either side of a long magnet and the difference in field strength observed at either end. The graph, produced in Python, depicts the intensity of the magnetic field in a 2D plane of the shield that displayed the strongest shielding properties (one of dimensions 40x60x1.5mm). The field strength is portrayed as a logarithmic scale of the actual values for the sake of clearer colour differentiation, and the field values were measured using a 3-axis Hall probe. These values were taken at regular intervals using 2D gridded paper as a reference.

 

It is worth noting that the slight leakage shown on the bottom of the shield could be explained by damage possibly being obtained during its creation; the asymmetry suggests it is not an intrinsic property of the shield. However, despite this, a reasonable amount of leakage is shown regardless of any damage, with a large radial field being observed at either end of the shield. This describes only a limited field channeling effect from the shield, and certainly does not prevent field lines flowing from one pole to the other. After this setup was measured, brief tests with the Hall probe also showed an increase in the field of the shield itself at an average of ~5%. This suggests that the shield was slightly magnetized by the close exposure in the experiment.

 

 

 

Whilst experimentation suggested completely shielding each pole from the other was impossible, practical application could still be found if it were possible to shield the effects of just one magnetic pole whilst leaving the other unchanged. This would involve creating a shield resembling an open box in which only one end of a bar magnet would reside, allowing the other to be relatively unaffected by the shield. To reduce the possibility of a strong field affecting the shields again, the magnet strength used in this part of experimentation was reduced to ~0.05T.

The investigation undertaken used a cylindrical MuMetal® shield of dimensions 35x26x1.5mm, closed at one end, which again required the Hall probe to measure the field strength. Observations are similarly displayed in Python:

 

 

 

 

 

 

 

 

 

 

 

 

The results displayed show that the obvious attraction between the shield itself and the magnet is almost as great as between the unshielded magnets. Like any other ferrous material, MuMetal® is always attracted to the source of the field. This makes shielding unhelpful for devices in which no interaction from the shielded pole is desired. The force between the magnet and the shield must also be overcome if the exposed side is to be directed towards the magnet (i.e. the basic function of a switch designed to alternate between shielding and exposing two magnets). This would require an energy input into the system that could use mechanical energy to turn the shield.

Further investigation was put into the idea of increasing the spacing between magnets as a way of creating a more effective shielding effect. The space was increased from 5.6cm to 9.8cm to observe the effect:

 

 

 

 

 

 

 

 

 

 

 

As predicted, there is less of an attraction between the shield and magnet at a greater distance. However, this drop in field strength closely mimics that of the separated, unshielded magnets. This suggests that the reduced potential between the shield and magnet is mainly down to the way the magnetic field strength falls away with distance, rather than a better shielding effect; again, the potential energy between the shield and the magnet is not much less than when both magnets are exposed.

From results obtained at Magnetic Shields Ltd., it can be said that a magnetic shield is still not a suitable method of completely shielding one pole of a magnet from the other. It also stands that magnetic monopoles are non-existent, and that shielding is again not a solution. Despite such limitations, the useful role of MuMetal® shielding for applications in which field distortion or confinement is necessary cannot be stressed enough. Figure 4 in the post Magnetic Shielding depicts some typical shielding factors obtained by smaller shields containing strong magnets, whilst some larger shields that contain weaker fields reach factors many times this magnitude.

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