21.
Phoenix Contact SMS relay
(www.phoenixcontact.com)
22.
Maxwell's Equations Explained Intuitively
(www.maxwells-equations.com)
23.
AB Circuit Protection Methods - Whitepaper
(literature.rockwellautomation.com)
24.
Putt's Law
(en.wikipedia.org)
25.
Advanced Ladder Logic Function (pdf)
(www.infoplc.net)