NEC POWERMATE ES User Manual page 129

Clean the mouse as follows.
5.
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Gently put the ball back into the mouse.
6.
Fit the ball cover back into the mouse and turn the cover clockwise
7.
until it locks in place.
5-10 Solving System Problems
Use tap water, or tap water and a mild detergent, to clean the
mouse ball.
Use a clean, lint-free cloth to dry the ball.
Blow into the mouse socket to remove remaining dust or lint.
Use a Q-tip to remove accumulations of dirt.