The inductance of an arc discharge has been modeled and a mathematical formula given for the arc inductance in terms of its geometrical dimensions. This formula agrees with the experimental results and is used to determine the arc‐channel radius. Inductance measurements have been taken for various values of pressure, interelectrode distance and applied voltage in a triggered spark gap. The dependence of inductance upon these parameters has been explained through the existing variations of the arc‐channel cross section. Two mechanisms are responsible for the variations of inductance. The first is diffusion and the second is the tendency of the current channel to vary diameter with pressure, interelectrode distance, and applied voltage. The time history of the arc‐channel inductance has been investigated. Finally, the ambipolar diffusion coefficient and the total charge number of the arc channel has been obtained.