Use of a permanent magnet for magnetic particle inspections

Permanent magnets produce a longitudinal magnetic field between the poles. Modern variants of the horseshoe magnet have adjustable arms, and may have variable geometry removable pole ends. Optimum defect detectability is at 90° to the poles. Modern opinion tends not to favour permanent magnets.

Advantages	Disadvantages
No power supply needed	Direct field only
Cling to vertical surfaces	Deteriorate with wear
No electrical contact problems	Have to be pulled from test surface
Inexpensive	No control over field strength
No damage to testpiece	Magnetic particles attracted to poles
Lightweight	Legs must have area contact
	May have to be recharged

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What are the advantages and disadvantages of utilising a permanent magnet for magnetic particle inspections?