Patents in the US last 20 years. Apparently this is a relatively common period for much of the world as well.
How was this period chosen? Did the US pick a period first and the rest of the world followed suit?
Do all of the original reasons for choosing that period still apply?
Was this chosen at the same time/era as the original length of copyright and were these periods previously or originally linked?
You have a lot of interesting questions, and the subject of patent term length is actually very important to the tech industry.
I would suggest the paper An Empirical Study of the Twenty-Year Patent Term by Mark Lemley (he's a huge scholar in the field of patent law). The paper itself will answer many of your questions; if not, you will most likely find the answers in one of many citations.
The book Privilege and Property also provides some interesting context for the early patent rules.
I can't provide historical context for the reason 20 years is the term for utility patents, but I can say that it didn't originate in the US. Up until June 8, 1995 the term for US patent was 17 years from the grant date. In an effort to align with the system from most other countries, the US adopted the 20 years from application date standard. Since it commonly takes about 3 years to prosecute a patent, the effective length of term is about the same. Indeed, if the patent prosecution takes an especially long time, a patent term extension may be allowed. Wikipedia has an article which provides some information.
Patents were systematically granted in Venice as of 1450, where they issued a decree by which new and inventive devices had to be communicated to the Republic in order to obtain legal protection against potential infringers. The period of protection was 10 years. These were mostly in the field of glass making. As Venetians emigrated, they sought similar patent protection in their new homes. This led to the diffusion of patent systems to other countries.
The Statute of Monopolies (1624) and the British Statute of Anne (1710) full title "An Act for the Encouragement of Learning, by vesting the Copies of Printed Books in the Authors or purchasers of such Copies, during the Times therein mentioned", was the first copyright statute and are seen as the origins of patent law and copyright respectively and firmly establishing the concept of intellectual property.
Different nations adopted to intellectual properties protection law gradually. USA adopted The Patent and Copyright Clause of the United States Constitution as early as 1787 and passed the legislation in 1790 titled "An Act to promote the progress of useful Arts.”
Initially, under the 1790 Patent Act the term could not exceed 14 years. but in 1836, Congress passed the Patent Act, which amended the statute to provide a term that could last for 21 years by providing for a 7 year extension from and after the expiration of the first term, which was modified in 1861, to 17 years from grant or twenty years from filing date.
In 1994 as US signed URAA (Uruguay Round Agreements Act) the patent term was amended to twenty years from the filing date of the earliest of such application.
