photoelectromotive

The story

1. Proto-Indo-European*bʰeh₂-
2. Proto-Indo-European*bʰéh₂os
3. Proto-Hellenic*pʰáwos
4. Ancient Greekφᾰ́ος (phắos)
5. Ancient Greekφῶς (phôs)
6. Ancient Greekφωτο- (phōto-)
7. Englishphoto-
8. Frenchélectro-
9. Ancient Greekἤλεκτρον (ḗlektron)
10. Latinēlectrum
11. Latinelectro-
12. Englishelectro-
13. Anglo-Normanmotif
14. Middle Frenchmotif
15. Proto-Indo-European*m(y)ewh₁-
16. Proto-Indo-European*m(y)owh₁éyeti
17. Proto-Italic*moweō
18. Latinmoveō
19. Latinmōtus
20. Late Latinmōtīvum
21. Middle Englishmotif
22. Englishmotive
23. Englishelectromotive
24. English todayphotoelectromotive

From photo- + electromotive.