In a charge potential evaluation device, the measured value of a potential difference Vc in a charged plate monitor (CPM) is converted into a potential difference Vh between the conductive pattern and load beam in a head gimbal assembly (HGA), using the following expression (1),

##EQU1##

where dh denotes the distance between the conductive pattern and the load beam in the HGA, dc denotes the distance between the conductive plate and a grounded surface in the CPM, h denotes the relative permittivity of an insulating foundation layer in the HGA, and c denotes the relative permittivity of the region between the conductive plate and the grounded surface in the CPM.

 
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