- I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.
- Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73; quoted in Encyclopaedia of Occupational Health and Safety (1998) by Jeanne Mager Stellman, p. 1992
- There cannot be a greater mistake than that of looking superciliously upon the practical applications of science. The life and soul of science is its practical application; and just as the great advances in mathematics have been made through the desire of discovering the solution of problems which were of a highly practical kind in mathematical science, so in physical science many of the greatest advances that have been made from the beginning of the world to the present time have been made in earnest desire to turn the knowledge of the properties of matter to some purpose useful to mankind.
- Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73, as quoted in The Life of Lord Kelvin (1910) by Silvanus Phillips Thompson
- Quaternions came from Hamilton after his really good work had been done, and though beautifully ingenious, have been an unmixed evil to those who have touched them in any way.
- Letter to R. B. Hayward (1892), as quoted in Energy and Empire : A Biographical Study of Lord Kelvin (1989) by Crosbie Smith and M. Norton Wise
- I am afraid I am not in the flight for “aerial navigation”. I was greatly interested in your work with kites; but I have not the smallest molecule of faith in aerial navigation other than ballooning or of expectation of good results from any of the trials we hear of. So you will understand that I would not care to be a member of the aëronautical Society.
- As a response to Major B. F. S. Baden Powell's request to join the Aeronautical Society, December 8, 1896.
- Often reproduced out of context and without citation to any primary source as "Heavier-than-air flying machines are impossible", like in The Experts Speak : The Definitive Compendium of Authoritative Misinformation (1984) by Christopher Cerf and Victor Navasky, p. 236
- Symmetrical equations are good in their place, but 'vector' is a useless survival, or offshoot from quaternions, and has never been of the slightest use to any creature.
- Letter to G. F. FitzGerald (1896) as quoted in A History of Vector Analysis : The Evolution of the Idea of a Vectorial System (1994) by Michael J. Crowe, p. 120
- Do not imagine that mathematics is hard and crabbed, and repulsive to common sense. It is merely the etherealization of common sense.
- Quoted in Life of Lord Kelvin (1910) by Silvanus Phillips Thompson
- Tesla has contributed more to electrical science than any man up to his time.
- Statement of 1896, as quoted in Prodigal Genius : The Life of Nikola Tesla (2007) by James J. O'Neill
- To live among friends is the primary essential of happiness.
- Lord Kelvin’s Replies to Addresses given on the Celebration of the Jubilee of his Professorship (June 15-17, 1896). Quoted in Lord Kelvin, Professor of Natural Philosophy in the University of Glasgow 1846-1899 by George F. Fitzgerald, 1899. Full Text
External Links 
- MacTutor Profile at University of St. Andrews
- William Thomson's Biography
- Lord Kelvin online
- Kelvin is Lord! - satirical website devoted to the "worship" of Lord Kelvin
- "William Thomson: king of Victorian physics" by Mark McCartney in Physics World (1 December 2002)
- Measuring the Absolute: William Thomson and Temperature (2002) by Hasok Chang and Sang Wook Yi (PDF file)
- The Molecular Tactics of a Crystal (1894)
- Baltimore lectures on molecular dynamics and the wave theory of light (1904)
- Mathematical and physical papers Volume V (1911)
- Treatise on natural philosophy Vol. 1 (1912 - 1923)
- Treatise on natural philosophy (Vol. 2)