Dry powder xerographic marking systems are capable of high quality printing but there is still need to improve their capabilities to better compete in offset markets. Many of these systems use two component magnetic brush technology to develop latent electrostatic images. The electrical properties of the developer material which makes up the magnetic brush play a large role in the quality of the developed image. Our desire to improve image quality characteristics affected by development has led us to explore the magnetic brush in more detail.We have used a high resolution electric field probe to characterize the dielectric constant and conductivity of a two component developer in a magnetic brush. Standard techniques use large area cells which look at the integrated properties of the developer material and cannot resolve variations at the spatial scales that are relevant for image uniformity. Magnetic brush structure is likely to translate into local electric field variation during the development process producing variations in toned image density on the photoreceptor and ultimately in the final printed image. Variations in electrical properties due to position in the development zone and magnetic field are looked at and discussed in the context of image quality. Realistic particle simulations are compared to experimental data.