I'm not sure what Faraday understood by 'imagination' (I'm sure a web search would help) but these definitions might help:
akin -> of the same kind as/like
idle -> in the sense of 'not having anything productive to do' - this meaning is preserved in the collocation 'idle curiosity': if you ask a question 'out of idle curiosity' you're not lazy or work-shy - you just have no object other than to satisfy your curiosity
wool-gathering->wandering about fields, picking up bits of wool that have fallen off sheep
yielding -> not in the sense of surrendering, or in the sense of responding passively, but in the sense of delivering something profitable.
little -> not much
Faraday thought that the 'fruits' of imagination were fun, but that they weren't worth the effort: 'Imagination...yield[s] pleasure, but little else'.
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