Discovery of Childhood and Invention of Universal Schooling
Instead of suggesting a device, I nominate the educational process essential for a high velocity of inventiveness, the evolution of a technological society, and the spread of culture. While schools for the elite have existed since antiquity, the recognition of childhood as a unique time of life with special schooling, social, and emotional needs, and different standards of justice, is relatively recent and associated with Rousseau, Freud, Piaget, and their forbearers.
The discovery that children are not "miniature adults" led to a more humane society and was essential to tailoring educational programs to the developmental stage of the student. Universal schooling (and even the modern university) were born both of this increased appreciation of the special needs of children and necessity — the industrial revolution needed a cadre of trained workers, scientists and engineers. The complexity of modern technology and the associated acceleration of innovation demand a critical mass of creative minds and hands that cannot be provided by occasional virtuosi toiling in solitude.