Electromagnetic Momentum, Energy and Mass, F. Rohrlich, Am. J. Phys. 38(11), November 1970
Electromagnetic systems of finite mass (m > 0) and of zero mass (m = 0) are considered. For both cases the electromagnetic energy and momentum are computed and are shown to lead to essentially different formulas. An invariant expression for the electromagnetic mass in m > 0 systems is derived, All m > 0 results are then specialized to the electrostatic case. Historical and present day confusion in the literature is discussed.