The magnetic Prandtl number { Pr _ { M } } is the ratio of viscosity to resistivity .
In astrophysical disks the diffusion of angular momentum ( viscosity ) and magnetic fields ( resistivity ) are controlled by turbulence .
Phenomenological models of the evolution of large scale poloidal magnetic fields in disks suggest that the turbulent magnetic Prandtl number { Pr _ { M,T } } controls the rate of escape of vertical field from the disk ; for { Pr _ { M,T } } \leq R / H vertical field diffuses outward before it can be advected inward by accretion .
Here we measure field diffusion and angular momentum transport due to MHD turbulence in a shearing box , and thus { Pr _ { M,T } } , by studying the evolution of a sinusoidal perturbation in the magnetic field that is injected into a turbulent background .
We show that the perturbation is always stable , decays approximately exponentially , has decay rate \propto k ^ { 2 } , and that the implied { Pr _ { M,T } } \sim 1 .