(source)

Baron William Thomson Kelvin (26 Jun 1824 - 17 Dec 1907) Irish physicist, mathematician and engineer , born as William Thomson in Ireland, he became an influential physicist, mathematician and engineer who has been described as the Newton of his era. Short biography of Baron William Thomson Kelvin >>

William Thomson Kelvin
“Measure … and express in numbers”

Illustrated Quote - Medium (500 x 250 px)

Click here or image for larger picture

Kelvin's Quadrant Electrometer (source)

“When you can measure what you are speaking about, and express it in numbers, you know something about it.”

— William Thomson Kelvin

From lecture to Institution of Civil Engineers (1883).

More William Thomson Kelvin quotes on science >>

Lord Kelvin delivered a lecture on “Electrical Units of Measurement” to the Institution of Civil Engneers on 3 May 1883, one of a series of six on “The Practical Applications of Electricity.” Since his topic concerned units, which are essential to measurement, Kelvin began this lecture by saying:

“In physical science a first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. 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 measure it, 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.”

This idea is often seen quoted in a condensed form: “If you cannot measure it, then it is not science,” though Kelvin himself left no record of expressing it in these few words.

It was in this same lecture that Kelvin remarked:

“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 the earnest desire to turn the knowledge of the properties of matter to some purpose useful to mankind.”

click for more about this quote and a larger picture • 500px • 800px

click for more about this quote and a larger picture • 500px • 800px

click for more about this quote and a larger picture • 500px • 800px

Nature bears long with those who wrong her. She is patient under abuse. But when abuse has gone too far, when the time of reckoning finally comes, she is equally slow to be appeased and to turn away her wrath. (1882) -- Nathaniel Egleston, who was writing then about deforestation, but speaks equally well about the danger of climate change today.

Carl Sagan: In science it often happens that scientists say, 'You know that's a really good argument; my position is mistaken,' and then they would actually change their minds and you never hear that old view from them again. They really do it. It doesn't happen as often as it should, because scientists are human and change is sometimes painful. But it happens every day. I cannot recall the last time something like that happened in politics or religion. (1987) ...(more by Sagan)

Albert Einstein: I used to wonder how it comes about that the electron is negative. Negative-positive—these are perfectly symmetric in physics. There is no reason whatever to prefer one to the other. Then why is the electron negative? I thought about this for a long time and at last all I could think was “It won the fight!” ...(more by Einstein)

Richard Feynman: It is the facts that matter, not the proofs. Physics can progress without the proofs, but we can't go on without the facts ... if the facts are right, then the proofs are a matter of playing around with the algebra correctly. ...(more by Feynman)

Quotations by: • Albert Einstein • Isaac Newton • Lord Kelvin • Charles Darwin • Srinivasa Ramanujan • Carl Sagan • Florence Nightingale • Thomas Edison • Aristotle • Marie Curie • Benjamin Franklin • Winston Churchill • Galileo Galilei • Sigmund Freud • Robert Bunsen • Louis Pasteur • Theodore Roosevelt • Abraham Lincoln • Ronald Reagan • Leonardo DaVinci • Michio Kaku • Karl Popper • Johann Goethe • Robert Oppenheimer • Charles Kettering  ... (more people)

Quotations about: • Atomic  Bomb • Biology • Chemistry • Deforestation • Engineering • Anatomy • Astronomy • Bacteria • Biochemistry • Botany • Conservation • Dinosaur • Environment • Fractal • Genetics • Geology • History of Science • Invention • Jupiter • Knowledge • Love • Mathematics • Measurement • Medicine • Natural Resource • Organic Chemistry • Physics • Physician • Quantum Theory • Research • Science and Art • Teacher • Technology • Universe • Volcano • Virus • Wind Power • Women Scientists • X-Rays • Youth • Zoology  ... (more topics)