Measurements are described of the potential variation of fine ribbon probes embedded in standard barium and strontium oxide coatings. Observations were made with 19 tubes at values of pulsed emission as high as 50 amperes/cm2. Results were as follows: (a) values of resistance were found ranging from 2 to 230 ohms‐cm2; (b) potential gradients were adequate to admit dielectric breakdown as a cause of sparking; (c) the curve of resistancevs. emission passes through a maximum and, at different temperatures, this maximum occurs at the same probe potential; (d) with BaO or mixed cathodes of higher values of resistance the resistance is largely at the interface, for low resistance coatings no observations on this matter were obtained; (e) the two best tubes as regards high emission without sparking had the lowest values of coating resistance; (f) the resistance of SrO cathodes is an order of magnitude higher than that of BaO or mixed cathodes; (g) superposition of d.c. upon pulsed emission gives a pronounced decrease of resistance.