* Principles of Operation
* Demagnetising Large Bodies
* Demagnetising Long Magnetic Bodies
* Summary


* Detailed product description

Principles of Operation

The PDM series generates waveforms which, when analysed, have continuous mathematical derivatives avoiding discontinuous rates of change in the applied field and potentially impairing the demagnetising process.  This is achieved using the capacitance discharge technique with electronic control of the field frequency.

The microprocessor controlled PDM unit generates frequencies of less than 0.01 Hertz and up to 100 Hertz.  The magnitude of the resultant magnetic field is controlled by the microprocessor programme within the limits of the resultant inductance for any particular application.  For certain applications it may prove inappropriate to apply higher frequency waveforms.

The PDM microprocessor controller enables operators to programme demagnetising processes as well as maintaining a non-volatile memory of pre-programmed processes.

Depending upon the size, coercivity, relative permeability etc,  of a particular body, there will be an optimum demagnetisation process.  If the applied frequency is too high, demagnetisation will not occur.  If the applied rate of decay is too fast or too slow, demagnetisation will not be effective. If the applied frequency is too low then the overall process could prove prohibitively long.

To optimise the processing time it may be necessary to have a range of processes for a range of bodies being demagnetised.  The PDM range incorporates the flexibility to optimise these parameters.

Hirst Magnetic Instruments Ltd.  Precision Demagnetisers have standard control units with variable output powers which are configured specific to particular applications.   Please refer to the factory for your specific requirements.

Demagnetising Large Bodies

When applying a changing magnetic field to a conductive body eddy currents will be induced.   When the material has a relative permeability (mr) less than 1, this situation is made worse.  As a consequence, if a field of too high a frequency is applied to a conductive body, eddy currents will supply sufficient “shielding” to prevent the penetration of the magnetic field into the mass of the large body.

Demagnetising Long Magnetic Bodies

It is necessary to apply the demagnetising field along the whole length of the long body.  Applying a demagnetising field to a section or sections of a long body will produce a localised effect that will be dominated by residual effects in the rest of the body, thus reducing the effectiveness of the demagnetising process.


Although these general rules can be applied to many applications there are inevitably significant subtleties with any application that must be taken into account otherwise the intended demagnetisation can actually render a magnetisation and worsen the situation.  Once the design and method have been correctly determined for a given application it will then continue to operate correctly without a need for modification.

© 2004 Hirst Magnetic Instruments Ltd. Tesla House, Tregoniggie, Falmouth, Cornwall TR11 4SN. United Kingdom.
Telephone +44 (0) 1326 372734 | Fax +44 (0) 1326 378069

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2004 Hirst Magnetic Instruments Ltd. Tesla House, Tregoniggie, Falmouth, Cornwall TR11 4SN. United Kingdom.
Telephone +44 (0) 1326 372734 | Fax +44 (0) 1326 378069
European Union Regional Development Fund

Hirst Magnetic Instruments Ltd. received funding from the BIG2 Project funded from the ERDF as part of the European Structural and Investment Funds Growth Programme 2014-2020