When
$p$
is an odd prime, Delbourgo observed that any Kubota–Leopoldt
$p$
-adic
$L$
-function, when multiplied by an auxiliary Euler factor, can be written as an infinite sum. We shall establish such expressions without restriction on
$p$
, and without the Euler factor when the character is non-trivial, by computing the periods of appropriate measures. As an application, we will reprove the Ferrero–Greenberg formula for the derivative
$L_p'(0,\chi )$
. We will also discuss the convergence of sum expressions in terms of elementary
$p$
-adic analysis, as well as their relation to Stickelberger elements; such discussions in turn give alternative proofs of the validity of sum expressions.