## Abstract

The thickness of anodic oxide films on chemically polished tantalum was determined from the wavelengths of the minima in the specular reflectivity using a value of the refractive index of 2$\cdot $20 $\pm $ 0$\cdot $02 at 5900 angstrom which was measured on detached flakes of the oxide by the immersion method. An auxiliary measure of increments of thickness was required for the analysis of the spectrophotometric measurements. This was provided by the quantity (Q$\Delta $(1/C))$^{\frac{1}{2}}$, where Q was the charge which was required to form the increment of thickness, and $\Delta $(1/C) was the corresponding increase in the reciprocal capacity. This measure of thickness, like that provided by the colours, is independent of the area. The analysis gives the refractive index between 2800 and 6000 angstrom, the net phase change in the two reflexions (with certain assumptions), and a value of $\epsilon /\rho $, where $\epsilon $ is the effective dielectric constant under the conditions used, and $\rho $ is the density. $\rho $ was determined by weighing specimens in air and water. The value obtained was 7$\cdot $93 $\pm $ 3%, which gives $\epsilon $ = 27$\cdot $6 $\pm $ 5%, at 1 kc/s. The effective surface area was then calculated, and was found to be very little different from the apparent area. The field strength during the formation of oxide at 9$\cdot $55 mA/cm$^{2}$ and 25$\cdot $8 degrees C was found to be 6$\cdot $61 $\times $ 10$^{6}$ V/cm and to be constant within experimental error, independent of thickness.