Controversy

As with many major inventions, there is some dispute as to whether Watt was the original sole inventor of some of the numerous inventions he patented. There is no dispute, however, that he was the sole inventor of his most important invention, the separate condenser. It was his practice (from around the 1780s) to pre-empt others' ideas which were known to him by filing patents with the intention of securing credit for the invention for himself, and ensuring that no one else was able to practice it. As he states in a letter to Boulton of 17 August 1784:

I have given such descriptions of engines for wheel carriages as I could do in the time and space I could allow myself; but it is very defective and can only serve to keep other people from similar patents.

Some argue that his prohibitions on his employee William Murdoch from working with high pressure steam on his steam road locomotive experiments delayed its development. Watt, with his partner Matthew Boulton, battled against rival engineers such as Jonathan Hornblower who tried to develop engines which did not fall foul of his patents.

Watt patented the application of the sun and planet gear to steam in 1781 and a steam locomotive in 1784, both of which have strong claims to have been invented by his employee, William Murdoch. Watt himself described the provenance of the invention of the sun and planet gear in a letter to Boulton from Watt dated 5 January 1782:

I have tried a model of one of my old plans of rotative engines revived and executed by W. M[urdock] and which merits being included in the specification as a fifth method...

The patent was never contested by Murdoch, who remained an employee of Boulton and Watt for most of his life, and Boulton and Watt's firm continued to use the sun and planet gear in their rotative engines, even long after the patent for the crank expired in 1794.

Watt opposed the use of high-pressure steam, and many inventors such as Richard Trevithick pioneered such engines, although frequently running into patent infringement actions by Watt. Those more efficient steam engines would eventually displace Watt's engines, leading to another industrial revolution with the development of the steam locomotive.