A transport current flowing through a superconducting film may cause these vortices to move with a constant velocity v proportional to, and perpendicular to the transport current. Because of their proximity to the surface, and their sharp field divergence at their centers, Pearl's vortices can actually be seen by a scanning SQUID microscope. The characteristic length governing the distribution of the magnetic field around the vortex center is given by the ratio Λ Λ = 2 λ λ 2 / d , also known as "Pearl length," where d is the film thickness and λ λ is London penetration depth. Because this ratio can reach macroscopic dimensions (~1 mm) by making the film sufficiently thin, it can be measured relatively easy and used to estimate the density of superconducting electrons.