A magnetism generation device is attachable to a disc brake rotor adapter for a bicycle hub. The magnetism generation device comprises a magnetism generation portion and a holder. The holder has a first portion and a second portion. The first portion supports the magnetism generation portion. The second portion is releasably attached to the first portion with a portion of the disc brake rotor adapter disposed between the first and second portions while the magnetism generation device in an installed state on the disc brake rotor adapter.
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