In order to understand clearly the progress of the experiments, it remains to say a few words about the curious phenomenon of electrical inertia which is called _self-induction_. When we want to set up an electric current, we find a certain initial resistance which ceases as soon as the current begins. If afterwards we want to break the current, it tends to maintain itself, and we have just the same trouble to stop it as to stop a vehicle in motion. It is a matter of daily experience. Sometimes the trolley of a tramcar leaves for a moment the wire which conducts the current, and we then see sparks. Why? There was a current passing from the wire to the trolley, and if the trolley breaks away from the wire for a moment, leaving an interval of air which obstructs the passage of electricity, the current will not stop. It has been set going, as it were, and it leaps the obstacle in the form of a spark. This phenomenon is what we call self-induction. Self-induction—or “self” as the electrical workers call it—is a real inertia. The surrounding medium offers resistance to the force which tends to establish an electric current, and to that which tends to stop a current already set up; just as matter resists the force which tends to cause it to pass from rest to movement, or from movement to rest. There is, therefore, a real electrical inertia as well as mechanical inertia. But our cathodic projectiles, our electrons, are charged. When they begin to move, they start an electric current; when they come to rest, the current ceases. Besides mechanical inertia, then, they must also have electrical inertia. _They have, so to speak, two inertias; that is to say, two inert masses, a real and mechanical mass, and an apparent mass due to the phenomena of electro-magnetic self-induction._ By studying the two deviations, electric and magnetic, of the Beta rays of radium or of the cathode rays, it is possible to determine the respective parts of each of these masses in the total mass of the electron. The electro-magnetic mass due to the causes which we have explained varies with the velocity, according to certain laws which we gather from the theory of electricity. Hence, by observing the relation between the total mass and the velocity, we can see what part belongs to the real and invariable mass and what to the apparent mass of electro-magnetic origin. The experiment has been made repeatedly by physicists of distinction. The result of it is surprising: the real mass is _nil_, and the whole mass of the particle is of electro-magnetic origin. Here is something that is calculated to modify entirely our ideas of the essence of what we call matter. But that is another story. Physicists then asked themselves—this is what we were coming to, after clearing the way of various difficulties—whether the relation between the mass and the velocity of the cathodic projectiles was the same as that which we found in virtue of the Principle of Relativity.