As of now, parts have been received for a diffusion vacuum pump which is capable of pulling a vacuum to 0.000000001 Torr (roughly the same pressure experienced in Low Earth Orbit, LEO). This will be the driving force behind all brush bulb research, and allow the determination of which vacua standards is applicable to Tesla’s design. A prototype design for the bulb has been constructed, with aim to understand which design criteria is necessary for achieving the rotating brush action.
It is worthy to note the various Brush Bulb designs Tesla put forth between the year 1892 – 1919. Distinctions during this time vary with the inner sphere changing in diameter, to the different sealing methods of the thinner tube. As can be determined, there does not seem to be an overall difference in design throughout the years.
Concluding with these images, design of the Brush Bulb has only varied with the interpretation of the artist at the time of drawing. What must be taken into account while dealing with experimental design, is the required and optimum proportions of the tube, B, sphere, S, and the degree of vacuum exhaustion in the outer globe, L.