What does the purple progress bar mean for electric drills?

With the .15 release of Factorio, nuclear power has been added. With this, came uranium ore. I just began harvesting some uranium with my electric drill, and noticed that there is a purple progress bar:

On electric mining drills that aren't working on uranium, this bar isn't here. I don't think it has to do with the speed modules that are inserted either (the bar doesn't seem to move any differently with or without them). What does it indicate?

This is game version 0.15.5 if that helps.

Version 0.15 introduced a new line of upgrades, called Mining Productivity. Researching these upgrades have a similar effect to using Productivity Modules, in that each miner will have a purple bar that slowly fills up, producing an additional ore when it fills up.

I haven't tested to see if burner drills receive the upgrade as well, but all electric drills are affected.