Details for the construction of a flat plate streaming potential apparatus are presented. Measurements using this apparatus on fused silica slides are reported and analyzed on the basis of a Gouy-Chapman-Stern-Grahame (GCSG) model for the electrical double layer at the silica-aqueous solution interface. It is found that the GCSG model can only partly account for the experimental results. Possible reasons for the discrepancy between experiment and theory are discussed.