wire, or filament, is of charcoal. Why does not the heated charcoal burn up? Read §§ 26 and 37. The answer is that there is no air in the bulb; FIG. 156. The incandescent bulb is a device by which we get light from a wire heated whitehot by an electric current. the air has been pumped out, and the charcoal is really being heated in a vacuum. Have you ever heard the loud report made by a bursting bulb? It is due to the rushing of air into the empty space inside the bulb. In more modern forms of electric light bulbs the filament is made of one of the rare elements, tungsten (tŭng'stn). Examine one of these bulbs, and see how fine the filament is. Some modern, very bright bulbs are filled with nitrogen. Arc Lights. The large, bright electric lamps which are used for street lighting, and which give a bluish-white light, are arc lights. They consist of two carbon rods through which a powerful current is allowed to pass. The rods are a little distance apart: 1 of an inch or so; but the current is conducted by a layer of white-hot carbon vapor that forms between them; this is called the arc." It has probably the highest temperature man has been able to produce: about 3800° C. The rods have to be renewed from time to time, as they are gradually burned away by being heated to so high a temperature in the air. 236. How Does a Dynamo Produce an Electric Current? You probably remember that when you studied the geography and early history of our country you learned that many of the rivers which empty into the Atlantic Ocean have falls or rapids at a certain distance from the ocean. Can you find out what is the cause of this? At many of these falls or rapids the settlers built mills, and towns grew up around the mills. Can you name any such towns? Elec Generator of tricity Water Penstock Why were the mills built at the waterfalls? It was because the rushing water was a source of energy, which could be used to turn the great wheel of the mill and the machinery attached to the wheel. Years afterwards, when coal came into general use, men cared less and less for water power, and built their mills and factories at places where coal could be obtained cheaply and in large amounts. Name some such places. But as electric power has come into use, water power has become of great importance once more; for the energy obtained from a far-away stream. can be turned into an electric current, and when it is thus harnessed it can be sent long distances over wires, and made to do a great deal of the world's work. The same power can be obtained at Water Turbine Waste Water FIG. 157. The force of falling water, or water pressure, turns the turbine, and this moves the dynamo, or generator, producing a powerful current. the bottom of a dam, because the water at the bottom is forced out in a powerful stream by the pressure of the water above it (see Fig. 157). What is the machine that brings to us the energy of falling water? Of course there is, first of all, the water wheel, which revolves in a circle; then the water wheel turns a dynamo (dī'nă-mō), which pro duces, or generates, the current. We cannot learn all about the dynamo now, but we can get an idea of the principle by which it works. To understand the dynamo we must go back to the study of the electromagnet (see § 230). You will remember that in an electromagnet we have a coil of insulated wire and a core of soft iron inside it. If we pass a current through the coil of wire, the soft iron core becomes a temporary magnet, for the reason that the iron is in the magnetic field which surrounds the wire. Suppose we put a magnet into a coil of wire that has no current (see Fig. 158), can we reverse the charge, and actually produce a current? This is just what happens, as long as the magnet is in motion. Whether we push the magnet into the coil, or pull it out of the coil, as long as we keep it moving, there will be a current in the wire. The current is produced because the magnetic field of the magnet moves past the wires, or, what is the same thing, because the wires move through the magnetic field of the magnet. FIG. 158. When a magnet A dynamo, or generator (see Fig. 159), is a wonderful machine in which coils of insulated wire are made to revolve rapidly through the fields of powerful electromagnets. The coils are all attached to a single frame called the armature. The currents produced in the wire coils of the armature are collected in two large wires which form the poles of the dynamo. What is the real use of the water, or steam, or other source of power which runs the dynamo? Is it not to turn the armature rapidly in the fields of the electromagnets, so that the energy of motion may be turned into electric energy? What do you say of the debt we owe to Faraday, who made the first dynamo? What is the importance of the dynamo to modern living? Who was Faraday? (Courtesy of the Westinghouse Electric & Manufacturing Co.) FIG. 159. Generator in Municipal Plant, Eau Claire, Wis. The engine is run by steam and in its turn makes the coils of wire revolve in the magnetic fields of powerful electromagnets. 237. How Does Electricity Produce Motion? - How can the water pressing against the dam at Keokuk be made to move street cars in St. Louis? How can the power of falling water at Niagara turn the wheels of a factory in Rochester? The first part of the process is, of course, to change the water power into electricity. But we do not really want the electricity for itself; we want it for the things we can do with it. Perhaps we want heat and light; perhaps it is motion of a trolley car or a sewing machine or a grindstone. You will see at once that what we need is some way of turning the electric current into motion; that is, it must make some "wheels go 'round," somewhere. The machine for doing this we call an electric motor. The motor is really like a dynamo, but its work is the reverse of that of a dynamo. What does a dynamo do? It changes energy of motion into an electric current, does it not? The work of a motor is to change the electric current back into energy of motion. In a dynamo we move coils of wire (the armature) rapidly in the fields of great magnets and thus produce currents. In a motor we pass the current from an outside source into the armature and electromagnets, and so cause the armature to revolve with great speed. Whatever machine is attached to the armature can thus be made to revolve also. Trolley wire + Power House + Motors Track FIG. 160. - The trolley car is driven by motors which get their current from a distant power station, where some generator is changing motion into electricity. Compare the arrangement shown in Fig. 160 with that of a real trolley car. The motor is on the under side of the car. The current enters through the trolley wire and pole. Sometimes a "third rail" carries the current from the power house to the motor of the car. The cur |