While the previous two editions use Gaussian units, the third uses SI units, albeit for the first ten chapters only. Jackson wrote that this is in acknowledgement of the fact virtually all undergraduate textbooks on electrodynamics employ SI units and admitted he had "betrayed" an agreement he had with Edward Purcell that they would support each other in the use of Gaussian units. In the third edition, some materials, such as those on magnetostatics and electromagnetic induction, were rearranged or rewritten, while others, such as discussions of plasma physics, were eliminated altogether. One major addition is the use of numerical techniques. More than 110 new problems were added.