level than the ocean, and the purpose of their flowing is to produce equilibrium, or an equality of level. Heat circulates in our rooms because a higher temperature exists in the fire than in the walls. In the electric lamps before me there is a very high temperature, because the carbon is maintained in a very high state of incandescence, and on the approach of my hand, which is of a lower temperature, toward the lamp, instantly there is a passage of heat from the high to the low temperature, and I feel that heat striking my hand exactly in the same way that I feel the draught (before mentioned) on my shoulders. Lastly, take electricity. We can not speak of the level of electricity; we can not speak of the pressure of electricity, in the same sense that we can speak of the pressure of liquids. We can not speak of the temperature of electricity; but there is a peculiar condition of electricity analogous to all three, analogous to pressure, temperature, and level, and that peculiar quality or condition is called potential. When we speak of electricity of a high potential, it has exactly the same meaning as high level of water, or of high temperature, and wherever two differences of potential are produced, separated by a conductor, electricity flows. Having said so much, perhaps too much, I want to speak to you about the production of electricity. Having shown you the various forms of energy, how it is transferred, and why it is transferred, I want now to say something of the production of electricity. You know very well that a lump of coal is a store of energy; that is evident by placing it in the fire. There are certain contrivances by which electricity can be produced from coal, but the effects are difficult to show to an audience. So we will take zinc, which, for the purpose, is better than coal. Zinc is a grand store of energy in its potential form. Here is a piece which I take up and place in a glass jar, and in that same glass jar I put a piece of carbon. I then place water in the jar, and, if I added to the water a little sulphuric acid, I should produce a galvanic battery; there would be a difference of potential, of which I just spoke, between the zinc and the carbon, which leads to the production of a current of electricity when the zinc and carbon are joined by a conductor. I have forty of these cells, or a battery, down-stairs, and I have two wires connected with its two ends, now in my hands. I know my battery is all right because, you see, when I bring the ends of the wire into contact with each other sparks are produced, which show that electricity is passing. I will not explain those sparks to you to-day, because I wish to talk to you about electromagnetic effects of electricity and how they are utilized. The first is the production of magnetism. I have here an electro-magnet, which is simply a bar of iron enveloped by an insulated copper wire, and a poker. A poker can be utilized for a great many purposes, but my present purpose is to place it on the large electro-magnet before you. So long as the battery wires are not joined to the wire around the electro-magnet, and, consequently, no current is passing, you see that
