To give you a better conception of an electric current, let us go back of the galvanic cell to the electric machine. If both poles of the machine are attached to rods terminating in round knobs we can set the machine in action and keep up a steady stream of disruptive discharges that will, if their frequency is great enough, perform the function of a current, and we have dynamic electricity from a statical machine; when the acid of the galvanic battery breaks down a molecule of zinc, energy is set free, and in the battery we have what corresponds to a disruptive discharge of infinitesimal proportions. This discharge would have been immediately converted into heat energy if the copper element had been left out of the battery, but as it is, it impresses itself on the atomic "brick" next to it, which establishes a chain of atomic movement throughout the circuit. This may constitute, if you please, a line of electrical force. But as thousands of these disruptive discharges are taking place simultaneously as many different lines of force are established. You must not conceive of these chains of atoms as simply thrown down like the bricks and left lying there, but that the atom is active; that it has the power to pick itself up again in an infinitesimally short time and is again knocked down (following the illustration of the bricks) by the next discharge along its line or chain of atoms. If you could get a mental picture of this action you would see that the whole conductor is in a most violent state of atomic motion of a peculiar kind. At the same time a part of this electrical motion is being converted into a heat motion of the atoms, and finally it all returns to heat unless some of it is stored up somewhere as potential energy. If the current has driven a motor that has wound up a weight, a part is stored up in the weight, which has the ability to do work if it is allowed to run down. If it drives machinery as it runs down, the mechanical motion is the expression of the stored energy. When the weight has run down the energy will be represented by the heat created by friction of the journals of the wheels and pulleys and the heating of the air. If the weight is allowed to fall suddenly it will heat the air to some extent, but mostly the earth and the weight itself will be heated. If the source of energy (the battery) is great and the pressure high and the conductor is too small to carry the energy developed in the battery as electricity, heat is developed, and if the heat is sufficiently intense, light also. We have seen (Vol. II) that heat motion when it reaches a sufficiently high rate throws the ether into a vibratory motion that we call light. However, this vibratory motion of the ether is set up long before it reaches the luminous stage; in other words, there are dark rays of the ether. We find that the electro-atomic motions of a conductor have the power to impress themselves upon the ether. [Illustration: Fig. 1.