Hello,
For air coils you could have a look here:
http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html
This is only one of the links from the EDUCYPEDIA page on coils:
http://educypedia.karadimov.info/electronics/coilsrelay.htm
Bertus
For air coils you could have a look here:
http://info.ee.surrey.ac.uk/Workshop/advice/coils/air_coils.html
This is only one of the links from the EDUCYPEDIA page on coils:
http://educypedia.karadimov.info/electronics/coilsrelay.htm
Bertus