Inertia Speed comes out of Hurter and Driffield's work. The line extending down is basically Gamma. Hurter and Driffield felt that the truest reproduction happened on the straight-line portion of the curve. This was based entirely on the sensitometric concepts. They never did psychohysical testing. That's what makes the first excellent print test so important. It began to tie the technical aspects of the photographic process in with the psychological.
Originally Posted by Michael R 1974
I can't give you specifics on the Inertia Speed at the moment as I am at work, plus I've never played with the method. The use of inertia speed was discontinued sometime around 1918. Curve shape had too big of an effect on inertia speeds, and like with Gamma, it was difficult to accurately define the straight-line portion with some curve shapes.