Notation

$E_k$	Electric field required for conventional breakdown of air
$N$	Number density of gas molecules
$n_e$	Number density of electrons
$Q_d$	Charge of disk
$R_d$	Radius of a disk
$Z_d$	Height of disk above ground
$\sigma_d$	Charge density on disk
$\mu_e$	Mobility of electrons
$\nu_m$	Effective collision frequency of electrons with neutral molecules and atoms
$v_e$	Net drift velocity of electrons
$\sigma$	Total conductivity
$\sigma_e$	Electron conductivity component
$\sigma_i$	Ion conducitivity component
$\tau _r$	Relaxation time