What's the best edition of Jackson's Classical Electrodynamics?
Well first of all I'm not sure about what you mean by "the best edition". Second, I have not used formally Jackson for any of my EM courses (I'm still undergrad) but I've been through some parts of the book using its second and first edition. Many of the information that I will tell you is on the Introduction part of the 3rd edition.
You see, the main difference is that the 1st and 2nd have this chapter on Magnetohidrodynamics, which the 3rd edition doesn't contain. Then there's the issue with the units, the 3rd edition uses SI units for a large part of the book (unlike the earlier ones) but returns to Gaussian units in the last chapters (you may or may not find this a good thing, depending on which units you are more comfortable with, I think).
I don't remember everything that the Introductions mentioned but you can find them all three in the 3rd edition and then judge by yourself.
The 3rd edition is more up to date with respect to computational methods of calculating fields. The first two editions were written before computers were widely used in electromagnetics.
As for going back and forth in units, that is a skill that any physicist needs.
I liked the second edition best but that might be because I had it the longest. I did prefer the units in the third edition though. I do not remember why I did not like the third edition as much. I remember the first time I took the book out of the library it was the first edition. The second edition was red; the first was green, and the third was blue. I have even heard the books described as Jackson-Green; Jackson-Red, and Jackson-Blue.
For me it's the contrary. I like the 2nd edition best, because it uses Gaussian units. That's close to the perfect system of units, which is the Heaviside-Lorentz system, i.e., rationalized Gaussian units used in (theoretical) high-energy particle physics.
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