I forgot to add that from what I have read of Hurter and Driffield's work, the BZTS is more like their understanding of things. They dealt with numbers and equations relating speed, density and contrast to time, temperature and developing agent. Using their concepts, I can take the results of three different tests to determine contrast index, two at one temperature and the third at a different temperature, and derive an equation from which I can prepare a chart of CI vs time and temperature that is accurate enough for practical work. But I am by nature an engineer. If I were teaching the photographic process, I would feel obliged to adjust my teaching methods to feedback from my students.