Aza Arnold

Aza Arnold (October 4, 1788 – 1865) was an American machinist, inventor, millowner, and patent attorney. Born in Smithfield, Rhode Island, he was trained as a machinist during his youth, and in 1809 entered work at Samuel Slater's textile manufacturing equipment plant in Pawtucket. After a failed venture in woolen blanket production and several years of employment at another Pawtucket factory, he invented a device to separate wool during carding and began to experiment with differential gear trains, possibly inventing the device independently or borrowing the concept from contemporary clockmaking.

After building and operating a cotton mill in Great Falls, New Hampshire, he incorporated a differential gear train into a cotton roving frame, allowing the bobbins to progressively slow relative to the spindles, in turn producing higher-quality thread. He patented the gear train in 1823, but was unable to obtain redress for rampant patent infringements, with the technology rapidly spreading across the United States and Europe over the following years. After several years administering a manufacturing plant in North Providence, Rhode Island, he moved to Philadelphia to operate a print works. He moved to Washington, D.C., around 1850 and entered work as a patent attorney. He patented his final invention, a self-raking saw, in June 1856. He died in Washington in 1865, and was buried at a local Quaker cemetery.