was reminded, beginning to read John S. Rigden's new book, of a joke that pokes fun at how physicists model the world. Various consultants are investigating a dairy farm to determine how to increase its productivity. Each makes a detailed report until the last one, a physicist, comes to the podium, draws a big circle on the blackboard behind him and says, ''Let us imagine the cow is a sphere. . . .''

The world is complicated and interesting, and to make any sense of it, one can try to find simple models that may capture its essence. If predictions based on these models produce reasonable agreement with observations, successive refinements can be made, presumably to achieve a more and more accurate approximation to reality. Naturally, the easiest way to test simple models is to seek out the simplest systems one can find in the laboratory.

Hydrogen is the simplest atom in nature; as a result, physicists have paid extensive attention to it over the past century. As the physicist Richard Feynman once said, ''There's a reason physicists are so successful with what they do, and that is they study the hydrogen atom and the helium ion and then they stop.''

Rigden has written a compelling tribute to this simplest of all atoms, using it as a hook for much of the history of physics in the first half of the 20th century, and for a description of how the knowledge gained resulted in some of the more fascinating bits of technology in the latter half of the century.

We first learn precisely how hydrogen formed a prototypical ''spherical cow,'' which the earliest physicists who hoped to understand atoms -- and who eventually discovered the weird wonders of quantum mechanics in the process -- relied on as they began largely blind investigations into the fundamental structure of matter. Since there were then no microscopes that could unveil the structure of atoms, these minute entities remained mostly mathematical curiosities throughout much of the 19th century.

Once it was discovered that the electron is a component of atoms, however, it became clear that atoms were themselves complex structures. The clue to unraveling the mystery of atoms lay in a seemingly innocuous set of colors emitted by each element as it is heated up, absorbing and then releasing energy in the process. The set of colors -- a spectrum -- gives a unique fingerprint by which the underlying composition of systems can be determined. But the question that nagged physicists was: what causes this characteristic spectrum?

It is here that the beauty of hydrogen's simplicity makes itself known. Had physicists concentrated on the spectrum of light emitted by even the next lightest element, helium, we would probably still be -- forgive the pun -- in the dark. However, a simple mathematical relation between the frequencies of light emitted by hydrogen, first discovered in 1885 by a Swiss high school teacher, Johann Jakob Balmer, later led the renowned Danish physicist Niels Bohr to develop the first modern picture of atoms, and to lay the basis of quantum mechanics. That was ultimately fully elucidated by his disciples Werner Heisenberg and Erwin Schrodinger. Next, the merging of the two great developments of 20th-century physics, quantum mechanics and special relativity, led to an understanding of the fine details of hydrogen spectra, and that led to the discovery of the spin of the electron and the existence of antimatter.

Rigden, the director of special projects at the American Institute of Physics, makes the sensitive and remarkably successful interplay between theory and experiment throughout the 19th and 20th centuries very clear. He demonstrates elegantly midway through ''Hydrogen'' -- when one senses his own interest in the subject really begins to peak -- how minute disagreements between theory and experiment, which otherwise would have been completely ignored, had to be taken seriously precisely because of the underlying simplicity of the hydrogen atom itself. Indeed, at a time when many books and news reports describe speculative theories that hope to probe deep cosmic mysteries but so far have failed to touch base with a single observation or experiment, it is a pleasant change to find a book on a humble topic that demonstrates the remarkable beauty and subtlety of nature, and of the experiments scientists have developed to explore it.

As a colleague of mine in physics puts it, the merger of quantum mechanics and special relativity that has allowed us to compare theory and observation at the level of better than a part in a billion -- the most successful confrontation between theory and experiment known in science -- may not be a theory of everything, but it is at least a theory of something.

THIS succinct and readable volume is not completely blemish free. In places it reads like a textbook, referring in early chapters to material to be covered later, and reviewing previous chapters at the beginning of new ones. Occasionally, as in discussions of the cosmological origin and detection of the heavy cousin of hydrogen, the isotope deuterium, minor errors in science or history creep in. And the introduction of mathematical formulas on occasion to illuminate a point may scare off more timid readers.

These are annoyances, but not great ones. Rigden's easy narrative style provides one of the most accessible descriptions of the importance of laboratory experimentation in developing our current understanding of fundamental physics that I know of. Also, he demonstrates how theorists have at times led the way, sometimes with jumps of intuition, sometimes with reliance on fundamental notions like symmetry and sometimes with sheer stubborn persistence. Finally, readers will particularly benefit from seeing how such fundamental experiments can lead to extremely important practical technologies that the original experimenters may never have dreamed of.

For a picture of how physics really progresses -- with gritty details filled in, along with ingenious experiments and glimpses of physicists who push the forefronts of knowledge -- Rigden's brief ode to hydrogen is a refreshing alternative to some of the speculative musings dominating the physics sections in bookstores.