- 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.
- 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 Robert Baldwin 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.
- 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
- I need scarcely say that the beginning and maintenance of life on earth is absolutely and infinitely beyond the range of sound speculation in dynamical science.
- As quoted in The Life of Lord Kelvin (1910), by Silvanus Phillips, Volume 2, (2005 edition, . p. 866)
- It is conceivable that animal life might have the attribute of using the heat of surrounding matter, at its natural temperature, as a source of energy for mechanical effect . . . .The influence of animal or vegetable life on matter is infinitely beyond the range of any scientific enquiry hitherto entered on. Its power of directing the motions of moving particles, in the demonstrated daily miracle of our human free-will, and in the growth of generation after generation of plants from a single seed, are infinitely different from any possible result of the fortuitous concurrence of atoms.
- As quoted in The Life of Lord Kelvin (1910), by Silvanus Phillips, Volume 2, (2005 edition, . p. 1093)
- 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.
- It is impossible by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects. [Footnote: ] If this axiom be denied for all temperatures, it would have to be admitted that a self-acting machine might be set to work and produce mechanical effect by cooling the sea or earth, with no limit but the total loss of heat from the earth and sea, or in reality, from the whole material world.
- Mathematical and Physical Papers, Vol.1 p. 179 (1882) "On the Dynamical Theory of Heat with Numerical Results Deduced from Mr Joule's Equivalent of a Thermal Unit and M. Regnault's Observations on Steam" originally from Transactions of the Royal Society of Edinburgh, March, 1851 and Philosophical Magazine iv, 1852
- 1. There is at present in the material world a universal tendency to the dissipation of mechanical energy.
2. Any restoration of mechanical energy, without more than an equivalent of dissipation, is impossible in inanimate material processes, and is probably never effected by means of organized matter, either endowed with vegetable life or subjected to the will of an animated creature.
3. Within a finite period of time past, the earth must have been, and within a finite period of time to come the earth must again be, unfit for the habitation of man as at present constituted, unless operations have been, or are to be performed, which are impossible under the laws to which the known operations going on at present in the material world are subject.
- Mathematical and Physical Papers, Vol.1 p. 512 (1882) "On a Universal Tendency in Nature to the Dissipation of Mechanical Energy" originally from the Proceedings of the Royal Society of Edinburgh for April 19, 1852, also Philosophical Magazine, Oct. 1852
- If the water flow down by a gradual natural channel, its potential energy is gradually converted into heat by fluid friction, according to an admirable discovery made by Mr Joule of Manchester above twelve years ago, which has led to the greatest reform that physical science has experienced since the days of Newton. From that discovery, it may be concluded with certainty that heat is not matter, but some kind of motion among the particles of matter; a conclusion established, it is true, by Sir Humphrey Davy and Count Rumford at the end of last century, but ignored by even the highest scientific men during a period of more than forty years.
- There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.
- Although reportedly from an address to the British Association for the Advancement of Science (1900), the quote is only duplicated without citation to any primary source in various books, including Superstring : A theory of everything? (1988) by Paul Davies and Julian Brown; also in Rebuilding the Matrix : Science and Faith in the 21st Century (2003) by Denis Alexander. To be more credible, a source prior to the 1980s and close to 1900 is needed.
- Confusion may be due to Michelson who made a similar quote whilst mentioning Lord Kelvin: In 1894, Albert A. Michelson remarked that in physics there were no more fundamental discoveries to be made. Quoting Lord Kelvin, he continued, “An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals." 
- The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. I. The first came into existence with the undulatory theory of light, and was dealt with by Fresnel and Dr. Thomas Young; it involved the question, how could the earth move through an elastic solid, such as essentially is the luminiferous ether? II. The second is the Maxwell–Boltzmann doctrine regarding the partition of energy.
Quotes about ThomsonEdit
- According to Sir W. Thomson's theory of Vortex Atoms, the substance of which the molecule consists is a uniformly dense plenum, the properties of which are those of a perfect fluid, the molecule itself being nothing but a certain motion impressed on a portion of this fluid, and this motion is shewn, by a theorem due to Helmholtz, to be as indestructible as we believe a portion of matter to be.