Before breaking the glass nipple: the filaments glow but barely don't heat.
After breaking the nipple: the filaments heat a lot but don't glow....
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https://www.youtube.com/watch?v=K4MwvZS0fVo
Videos and infos about the Avery mark IV:
https://www.youtube.com/watch?v=l81pjhVVX_o
https://www.youtube.com/watch?v=QBg6OMuTqng
https://www.youtube.com/watch?v=5YdlyHZZXVc
User manual:
http://www.averybiomedical.com/wp-content/uploads/2017/11/EN-6025-AC-Instruction-Manual.pdf
My pacemakers playlist:
https://www.youtube.com/playlist?list=PLpnL3w7u1Hr02YJVg6Zgz7S9XW6_iExvP
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https://www.youtube.com/watch?v=Jvt2vyhaC-8