About twenty years ago, I asked one of my colleagues what was the unit “Dirac” for. He, a competent physicist, thought about it for a while in earnest and shyly replied that he did not know. My question was a joke borrowed from the following passage from Calder's book [Ref. 1]:
Dirac’s taciturn and retiring behavior are famous; in his days at Cambridge, a unit of volubility called a Dirac meant one word per year.
P. A. M. Dirac on a Mali Republic stamp issued in 2011
Reading a new biography [Ref. 2], written by Farmelo, of the Nobel-winning physicist P. A. M. Dirac, I found a different definition of the unit Dirac, i.e., one word per hour. This definition is more realistic than that in Ref. 1. However, the latter, which exaggerates Dirac's taciturnity, is funnier than the former and too good to be discarded.
The relevant description in Ref. 2 is quoted by Gilleland [Ref. 3] under the title “Unit of taciturnity.” The unit surely commemorates Dirac’s taciturnity. However, its proper name would be “unit of volubility.” The reason is that the number of words per unit time is smaller for the person of higher taciturnity.
References
N. Calder, The Key to the Universe (Viking, New York, 1977).
G. Farmelo, The Strangest Man: The Hidden Life of Paul Dirac, Quantum Genius (Faber and Faber, London, 2009).
M. Gilleland, “Unit of taciturnity: The Dirac.” Blog post at Laudator Temporis Acti (January 26, 2010).
Note: This short essay first appeared at https://ideaisaacjoking.blogspot.com/2010/01/unit-dirac.html on January 27, 2010, and has been one of the most viewed posts of the present author (2.56-k views). In the version given here, he improved English.