ALTERNATING CURRENT. For the sake of simplicity and convenience we speak of electricity as always flowing in through one wire and out through the other. With batteries this is actually the case. It is also the case where people have what is called _direct-current_ (d. c.) electricity. But it is easier to raise and lower the voltage (pressure) of the current if instead of being direct it is _alternating_; that is, if for one instant the electricity flows in through one wire and out through the other, the next instant flowing the opposite way, then the first way again, and so on. This kind of current is called _alternating current_ (a. c.), because the current alternates, coming in the upper wire and out of the lower for a fraction of a second; then coming in the lower and out of the upper for the next fraction of a second; then coming in the upper again and out of the lower for a fraction of a second; and so on, back and forth, all the time. For heating and lighting, this alternating current is just as good as the direct current, and it is probably what you have in your own home. For charging storage batteries and making electromagnets, separating water into two gases, and for running certain kinds of motors, however, the direct current is necessary. Find out whether the current in your laboratory is direct or alternating. _APPLICATION 49._ Explain why we need fuel or water to generate large currents of electricity; how we can get small amounts of electricity to flow without using dynamos; why automobiles must be cranked unless they have batteries to start them. INFERENCE EXERCISE Explain the following: 301. Mexican water jars are made of porous clay; the water that seeps through keeps the water inside cool. 302. When you crank an automobile, electricity is generated. 303. Potatoes will not cook any more quickly in water that is boiling violently than in water that is boiling gently. 304. When you brush your hair on a winter morning, it sometimes stands up and flies apart more and more as you continue to brush it. 305. You cannot see a person clearly through a ground-glass window, although it lets most of the light through. 306. There is a layer of coarse, _light-colored_ gravel over the tar on roofs, to keep the tar from melting. 307. It is very easy to slip on a well-waxed hardwood floor. 308. If you have a silver filling in one of your teeth and you touch the filling with a fork or spoon, you get a slight shock. 309. You can shake a thing down into a bottle when it will not slip down by itself. 310. If you rub a needle across one pole of a magnet three or four times in the same direction, then float it on a cork in water one end of the needle will point north. SECTION 34. _Conduction of electricity._ How does electricity travel? Why do you get a shock if your hands are wet when you touch a live wire?