A historical masterpiece that laid the foundation for classical physics
Laid the foundation for the emergence of modern physics in the 20th century

If classical physics is broadly divided into mechanics and electromagnetism, and mechanics is said to be based on Newton's Principia, then electromagnetism can be said to be based on Maxwell's Electromagnetism.
Moreover, this book vividly documents the process by which new concepts were formed to shape electromagnetism into what it is today, something you will never find in today's electromagnetism textbooks, and how mathematics was used to define those concepts.
Author James Clerk Maxwell (1831-1879) expressed the seemingly unrelated laws of electricity and magnetism that had been discovered over nearly a century into four partial differential equations for electric and magnetic fields, establishing Maxwell's equations, and revealed that these were the fundamental laws that govern all electromagnetic phenomena.
Furthermore, by theoretically predicting the existence of electromagnetic waves from Maxwell's equations and confirming that light is a type of electromagnetic wave, it inspired Einstein to formulate the special theory of relativity in the 20th century, and the subsequent theory of light directly contributed to the birth of quantum mechanics in the early 20th century.
In other words, Maxwell's Electromagnetism became an important foundation for the emergence of modern physics in the 20th century.

“Knowing the dimensions of a unit gives us a convenient way to check the correctness of the equations resulting from complex investigations.
The dimensions of each basic unit of such a complex equation must be the same.
If not, then the formula makes no sense and there must be something wrong somewhere.
Because the interpretation of the formula will vary depending on which system of units is used.”
---From "Before You Begin: Measurement of Physical Quantities, page 2"

“It may be considered that the law of force was established with considerable accuracy by Coulomb's experiments using a torsion balance.
However, this type of experiment is difficult and somewhat uncertain due to several intractable causes, so it is imperative to thoroughly track down and properly correct these causes.
First, the two objects must be reasonably sized relative to the distance between them, so that each object can carry a sufficient amount of charge for the force to be measured.
Then, the action of each object affects the charge distributed on other objects, so we cannot think of the charge as being evenly distributed over the surface of the object or concentrated at the center of gravity.
Rather, its effects must be calculated through complex research.
However, Poisson was very good at calculating problems involving two spherical objects, and W.
Sir Thomson greatly simplified the calculations by using the theory of image charges.
(……) The fact that an object is imperfectly insulated also presents its own difficulties, separate from other situations, because the object's charge continues to decrease.
Coulomb investigated the law of dissipation of electric charge and corrected it by taking into account the effects of dissipation of charge in his experiments.”
---From “Part 1, Chapter 1, On the Phenomenon, pp. 64-65”

“Now let us assume that object A is a conductor.
If we treat this as a case of equilibrium of a system of several bodies, and consider the movable charge as part of this system, we can argue that the system is unstable when we remove many degrees of freedom by fixing the charge, and that the system should be more definitely unstable when we regain those degrees of freedom.
However, we can consider this case in a more special way.
First, let us assume that the charge is fixed on A and that A moves a short distance dr.
The increase in the potential of A due to such causes has already been taken into account.
Next, let us assume that the charge is always allowed to move to a stable equilibrium position inside object A.
During this exercise, the potential must decrease by an amount that can be called Cdr.”
---From "Part 1, Chapter 6, Equilibrium Points and Parallel Lines, pp. 229-230"

“A galvanometer generally consists of one or more coils of silk-covered wire and a magnet suspended on a horizontal axis inside the coils.
When current flows through the wire, the magnet tries to orient itself on an axis perpendicular to the plane in which the coils lie.
“If the plane of the coils is parallel to the plane of the Earth's equator, and if the current flows through the coils from east to west, which is the direction of the apparent motion of the Sun, the magnets inside the coils will think that their magnetization is that of a giant magnet and will move in a direction that tries to align with the Earth's magnetization, similar to the point of a compass needle with the North Pole pointing south.”
---From "Part 2, Chapter 1, Current, p. 455"

“If a large number of Leyden bottles, each of which has a very large capacitance, are connected in series by conductors of very high resistance (such as the wet cotton in Gogain’s experiment), the electromotive force acting on this series connection will cause a current to flow, which the galvanometer indicates will gradually decrease until the Leyden bottles are completely charged.
The apparent resistance of such a series connection increases, and if the dielectric making up the Leyden bottle were a perfect insulator, the resistance would increase indefinitely.
Now, if we remove the electromotive force and connect the two ends of the series connection, we will observe a current flowing in the opposite direction, and in the case of a perfect insulator, the total charge due to such a current will be equal to the total charge due to the direct current.
A similar effect is observed in the secondary pile, with one difference being that although the final insulation is not as good, the capacitance per unit surface is vastly greater.
In the case of cables whose covering is an insulator such as gutta-percha, if an electromotive force is applied for half an hour and then the conductor is connected to an external electrode, a current in the opposite direction is generated, which flows for some time and the system gradually returns to its original state.
This phenomenon is similar to the phenomenon called 'residual discharge' in a Leyden jar, except that the amount of polarization is much greater in insulators such as gutta-percha than in glass.”

---From “Part 2, Chapter 12, On the Electrical Resistance of Materials, pp. 626-627”