Great Experiments in Physics [Paperback]

I strongly recommend this book for everyone interested in physics. Professor Shamos did a fantastic job in collecting in one single book the experiments and original works of the main geniuses of the history of physics. It's so much more interesting and easy to understand the principles when you visualize the context in which they were developed. Such literature must be obrigatory for every student of physics !!

Ok, so, this is a good book. If you are interested in the history of science or just physics, and you want to break into primary sources but feel a little overwhelmed, then I recommend going ahead and buying this book, because it's cheap, and flipping to the first chapter that catches your eye. You will probably like what you see: English translation of texts by Galileo, Boyle, Cavendish, etc., which are short excerpts of the "good part". In the margins, there are commentaries to help you out with archaic language and "weirder" aspects. Now you're cooking! You can go on to other reading if you want.

This is a haphazard sourcebook with mediocre, short introductions to each paper that are almost entirely biographical. Some of the selections are not really experiments at all, e.g. the usual excerpts from Galileo and Newton on mechanics and most of the 80-page appendices (Maxwell, Einstein, Bohr, etc.). Others are more measurements than "great experiments", e.g. Boyle, Coulomb, Cavendish. Isolated but interesting selections are Young and Fresnel on light (no Newton here), Röntgen on x-rays, Becquerel on radioactivity. The only reasonably coherent thread that one can follow through the book is electromagnetism. I shall summarise the main points briefly.

Coulomb (1785) discovered his "fundamental law by which electrified bodies repel each other", namely that the repulsive force is inversely proportional to the square of the distance, as is the attractive force between opposite charges. But this is the same law as for magnets. Thus "The magnetic fluid seems to have, if not by its nature, at least by its properties an analogy with the electric fluid. Based on this analogy it can be assumed that the two fluid obey the same laws. In all other phenomena of attractions or repulsion that nature presents to us, for instance elasticity and chemical affinity, the forces seem to be exerted only at very small distances, and it seems, therefore, that they are nothing but the same laws of electricity and magnetism."

The link between electricity and magnetism was further strengthened when Oersted (1820) discovered that a current can influence a magnetic needle. Apparently the current generates some sort of "electric conflict" which "is not enclosed in the conductor, but ... is at the same time dispersed in the surrounding space, and that somewhat widely". "All nonmagnetic bodies seem seem to be penetrable through electric conflict; but magnetic bodies, or rather their magnetic particles, seem to resist the passage of this conflict, whence it is that they can be moved by the impulse of contending forces."

Faraday (1832) discovered further that currents can induce currents (albeit weak ones: "I could obtain no evidence by the tongue") and that magnets can induce currents. As for the nature of this electromagnetic business, Faraday (1834) made some discoveries on electrolysis. Water can be decomposed into hydrogen and oxygen by sticking two metal plates in it and connecting them with a current, and Faraday discovered that "when subjected to the influence of the electric current, a quantity of it is decomposed exactly proportionate to the quantity of electricity which has passed" and thus "it seems probable, and almost a natural consequence, that the quantity which passes is the equivalent of, and therefore equal to, that of the particles separated; i.e., that if the electrical power which ... makes a grain of oxygen and hydrogen in the right proportions unite into water ... could be thrown into the condition of a current, it would exactly equal the current required for the separation of that grain of water into its elements again." So electricity seems to be of a material nature.

Other experiments (and the theory of Maxwell (1865)), however, makes electricity seem like a wave phenomena, as represented here by Hertz (1888): "the action of an electric oscillation spreads out as a wave into space" so "I planned experiments with the object of concentrating this action ... by putting the primary conductor in the focal line of a large concave parabolic mirror" and indeed "I have succeeded in producing distinct rays of electric force, and in carrying out with them the elementary experiments which are commonly performed with light and radiant heat". If such rays were material it ought to be possible to deflect them by magnetic fields, which could at first not be effected.

Then Thomson (1897) finally managed to set up an experiment where "cathode rays [that] carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which the force would act on a negatively electrified body moving along the path of these rays." Consequently, "I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter. The question next arises, what are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision? To throw some light on this point, I have made a series of measurements of the ratio of the mass of these particles to the charges carried by it." Basically, one measures the total amount of electricity in the ray by putting a sensor at the end of it, and the total kinetic energy by having the ray bump into an obstacle and measuring the increase in temperature; knowing the magnetic field, one can figure out the ratio mass/charge from these values. We find that this ratio is about 10^-7 which is "very small compared with the value 10^-4, which is the smallest value of this quantity previously known, and which is the value for the hydrogen ion in electrolysis."