Possible Clues:

• They're positively charged to achieve top mark, with knobs on!
• A bit of poetry is what makes them so positive (6)
• Positively a bit of poetry is like this
• They attract negative ions
• Current hot spots?
• Magnetron parts
• Sea don (anag.)
• Some poetry is enough to make them positive
• Electron tube ends
• Positive electrodes, perhaps