The cathode tubes of radium give out a continuous bombardment of these minute projectiles, charged, not with melinite, but electricity: far smaller than the shells of our artillery, but animated with infinitely greater initial speeds. The velocity of “Bertha’s” shells is contemptible in comparison.
But how was it possible to measure the speed of these projectiles?
We know that electrified bodies act upon each other. They attract or repel each other. Now our electrons are charged with electricity. If, therefore, we put them in an electric field, between two plates connected at the edges by an electrical machine or an induction coil, they will be subjected to a force that will cause them to change their direction. The cathode rays, in other words, will change their direction under the influence of an electric field. The amount of diversion will depend upon the speed of the projectiles and upon their mass; that is to say, upon the resistance of inertia which the mass opposes to the causes which tend to divert it.
But this is not all. The electric charges borne by the projectiles are in movement, even rapid movement. Now, electricity in movement is an electric current, and we know that currents are diverted by magnets or magnetic fields. Therefore the cathode rays will be diverted by the magnet. This diversion will, like the former, depend upon the velocity and the mass of the projectile; but not quite in the same way. Other things being equal, the magnetic diversion will be greater than the electrical diversion, if the velocity is high. As a matter of fact, the magnetic diversion is due to the action of the magnet on the current. It will be greater in proportion to the intensity of the current; and the current will be more intense in proportion to the height of the velocity, since it is the movement of the projectile which causes the current. On the other hand, the trajectory of our little projectiles will be less influenced by the electrical attraction in proportion as the velocity of the projectile is great.
Hence it is easy to see that when we subject a cathode ray to the action of an electric field, then to that of a magnetic field, we may, by comparing the two deviations, measure at one and the same time the velocity of the projectile and its mass (related to the known electric charge of the electron).
In this way we find enormous velocities, rising from a few tens of kilometres to 150,000 kilometres a second, and even more. As to the Beta rays of radium, they are still more rapid. In cases they attain velocities not far short of that of light, and higher than 290,000 kilometres a second. Here are just the velocities we need in order to test whether or no mass increases with them.
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.