RECENT WONDERS OF ELECTRICITY. 675 



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 elec- 

 tricity 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 trans- 

 ferred, 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 diffi- 

 cult to show to an audience. So we will take zinc, which, for the pur- 

 pose, is better than coal. Zinc is a grand store of energy in its poten- 

 tial 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, 

 clown-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 electro- 

 magnetic 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 



