„All this is very good in theory, but in practice, you take a piece of iron, wind a wire around it, then plug the wire in. The core gets hot, the wires smoke, and the fuse blows. So you see, there are practical limitations to theory.“

—  James Alfred Van Allen, Comments to an undergraduate physics class about transformers, Reach Into Space http://www.time.com/time/magazine/article/0,9171,892531,00.html, Time, 1959-05-04.
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„When a constant electric current flows along a cylindrical wire, its strength is the same at every part of the section of the wire. But if the current is variable, self-induction produces a deviation from this... induction opposes variations of the current in the centre of the wire more strongly than at the circumference, and consequently the current by preference flows along the outer portion of the wire. When the current changes its direction... this deviation increases rapidly with the rate of alternation; and when the current alternates many million times per second, almost the whole of the interior of the wire must, according to theory, appear free from current, and the flow must confine itself to the very skin of the wire. Now in such extreme cases... preference must be given to another conception of the matter which was first presented by Messrs. 0. Heaviside and J. H. Poynting, as the correct interpretation of Maxwell's equations as applied to this case. According to this view, the electric force which determines the current is not propagated in the wire itself, but under all circumstances penetrates from without into the wire, and spreads into the metal with comparative slowness and according to laws similar to those which govern changes of temperature in a conducting body.
... Inasmuch as I made use of electric waves in wires of exceedingly short period in my experiments on the propagation of electric force, it was natural to test by means of these the correctness of the conclusions deduced. As a matter of fact the theory was found to be confirmed by the experiments...“

—  Heinrich Hertz German physicist 1857 - 1894
"On the Propagation of Electric Waves by Means of Wires" (1889) Wiedemann's Annalen. 37 p. 395, & pp.160-161 of Electric Waves

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„Democracy is beautiful in theory; in practice it is a fallacy. You in America will see that some day.“

—  Benito Mussolini Duce and President of the Council of Ministers of Italy. Leader of the National Fascist Party and subsequent Republic... 1883 - 1945
to Edwin L. James of the New York Times (1928)

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„In theory, there is no difference between theory and practice. But, in practice, there is.“

—  Jan L. A. van de Snepscheut Dutch computer scientist 1953 - 1994
Attributed by Doug Rosenberg and Matt Stephens (2007) Use Case Driven Object Modeling with UMLTheory and Practice p. xxvii The earliest known appearance of this quote in print is Walter J. Savitch, Pascal: An Introduction to the Art and Science of Programming (1984), where it is attributed as a "remark overheard at a computer science conference". It circulated as an anonymous saying for more than ten years before attributions to van de Snepscheut and Yogi Berra began to appear (and later still to various others).

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„In theory there is no difference between theory and practice; in practice there is.“

—  Yogi Berra American baseball player, manager, coach 1925 - 2015
Attributed in Nassim Nicholas Taleb, Antifragile - Things that Gain From Disorder (2012), p. 213. The earliest known appearance of this quote in print is Walter J. Savitch, Pascal: An Introduction to the Art and Science of Programming (1984), where it is attributed as a "remark overheard at a computer science conference". It circulated as an anonymous saying for more than ten years before attributions to Jan L. A. van de Snepscheut and Yogi Berra began to appear (and later still to various others).

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