An equation was derived for the fraction Q of the incident x‐ray power P which escapes as kinetic energy of electrons. Q=0.140τS(1+e−μl)[r−1r(1−λλK)3+1r(1−λλL)3+1.20(1−wK)(r−1)r(λλKα−λλL)3], where τ and μ are, respectively, the linear true and total absorption coefficients for the incident wave‐length λ, S is the range of an electron having an energy hν corresponding to the wave‐length λ, r is the K absorption jump ratio and wK is the K fluorescent yield. λKα, λK and λL are, respectively, the wave‐lengths of the K lines, the K absorption limit and the L absorption limit. This equation can be used to eliminate the necessity of guard electrodes.