This shows the "parallel kinetic energy" K p on the horizontal axis and the total energy ε on the vertical axis. An electron inside the bulk metal usually has values of K p and ε that lie within the lightly shaded area. It can be shown that each element d ε d K p of this energy space makes a contribution z S f F D d ϵ ϵ d K p to the electron current density incident on the inside of the emitter boundary. Here, z S is the universal constant (called here the Sommerfeld supply density):