Requirements for two types of board for electrical purposes made from glass bonded mica powder, from $9B in (3.2 mm) up to $9H in (12.7 mm) in thickness. Definitions, classifications, finish, flatness, thickness, electric strength proof test in oil, surface electric strength proof test in air, insulation resistance, carbon arc test, spirit burner test, water absorption, deformation under compression, shear strength and cross breaking strength. Methods of test given in appendices.