stances together. Conductors can be electrified only if held by means of an insulator.

An electrified body attracts one not electrified, then charges it with its own kind of electric charge, and then repels it.

The space around a charged body is an electric field. An insulated body brought into the field is electrified by induction.

Generator Room of the Mississippi River Power Co. at Keokuk. A dam 4,649 feet long extends across the river, and raises the water behind the dam. Some of the water rushes through turbines, generating_200,000 horse power (cf. § 26) of electricity. About a third of this is carried to St. Louis, 137 miles away.

When the strain in the electric field between two oppositely charged bodies becomes sufficiently great, a discharge takes place, usually as a spark. Discharge takes place most easily from points.

An electric condenser consists of two conductors, such as metal foil, separated by a non-conductor, such as glass. The common form is the Leyden jar.

Franklin proved that lightning is a huge electric spark. His kite was electrified by induction from the charged cloud.

Electric currents are usually generated in cells or by dynamos. The simple cell consists of zinc, copper (or carbon), and dilute sulphuric acid. The current flows from zinc to copper inside the

liquid, and from copper to zinc outside of the liquid. A group of connected cells is a battery.

The simple cell is easily polarized by the hydrogen which collects upon the copper (or carbon) plate. Polarization is prevented, in Daniell's cell, by a layer of copper sulphate solution between the zinc and the copper. In other cells oxidizing materials are put into the solution, or mixed with the carbon. These convert the hydrogen into water.

Wires carrying currents are magnets. Electromagnets are formed when soft iron is put into coils carrying currents.

The telegraph is essentially an electromagnet, with a key for making and breaking the circuit.

The electric bell is an electromagnet in which the armature carrying the hammer continually breaks the circuit, causing the hammer to vibrate back and forth.

The resistance which the current meets in passing through conductors appears as heat. This is the principle of electric lighting and heating devices.

Electroplating is due to the electrolysis of solutions of the compounds of certain metals, such as copper, silver, gold, nickel, etc.

A dynamo is a device in which coils of wire, moving rapidly through the fields of powerful magnets, produce a current.

A motor is a dynamo reversed. In it an electric current is changed into mechanical motion.

Men get electric power on an enormous scale by changing the mechanical energy of falling water into electric currents.

164. Exercises.

1. If piece of steel is held near a dynamo, it is pulled strongly toward the dynamo. If held there for a short time, it becomes a magnet. Explain both facts.

2. Write in the Morse code: Will visit your school Tuesday, April 9. Leave short spaces between letters and longer ones between words.

3. Read an account of the life of Morse, the inventor of the telegraph.

4. What is meant by the terms "triple plate, quadruple plate," etc., as applied to silver utensils?

5. How could a slab of impure copper be purified by the electric current?

6. How could coal be converted into energy at the mine, and this energy distributed without the shipping of the coal?

7. When the lighting system of a trolley car is on the same circuit as the power, the lights often burn low while the car is being started, but burn brightly when the car is in motion. What does this show as to the power required to start the car as compared with that needed to keep it moving?

8. What advantages has electric power over steam power for factory use? For the home?

CHAPTER IX

LIGHT AND SOUND

165. Luminous Bodies. Every object that we see "is seen," or "is visible," because light from the object enters the eye. When the visible body itself produces the light, the body is said to be self-luminous. Self-luminous bodies are usually very hot. This is the case with the sun, a lamp flame, an arc electric light. However, most visible bodies are not self-luminous; but they shine, or are seen, by light which they receive, and then reflect to the eye. We say that non-luminous bodies are illuminated by luminous bodies. The moon is a non-luminous body. Moonshine is sunshine reflected to us from the moon's surface. An object seen by moonlight is thus seen by sunlight that has been reflected twice: first from the moon to the object, and then from the object to the eye. When we take a lamp into a dark room, we see the objects in the room, because they reflect lamplight to the eye.

166. Transparent and Opaque Bodies.- Substances through which objects are seen clearly are said to be transparent. Such are water, glass, and air. A substance that does not permit light to pass through it is said to be opaque. Wood and iron are opaque. Certain substances allow some of the light to pass, but objects cannot be seen distinctly through them; such substances are said to be

IMAGES THROUGH SMALL OPENINGS

151

translucent. Examples of translucent substances are fog, ground glass, oiled paper, and thin china. Very thin plates of all bodies, even metals, are translucent. Thus, gold foil transmits a green light.

167. Light and Its Properties. Light is the cause outside of us that produces the sensation of sight. Sunlight produces other effects besides the lighting of objects. Thus, if the skin is exposed to sunlight, it is not only illuminated, but also warmed. In addition, chemical changes are brought about in the skin, and it is tanned. The energy of the sun thus appears as heat, light, and chemical energy.

The velocity of light is very great: about 186,000 miles a second (cf. § 11). Sound waves travel through air at the rate of about 1,100 feet a second; but this is a snail's pace as compared with light. Because of the greater speed of light we see the flash of a gun before we hear its report, and we see the lightning long before we hear the thunder that accompanies it.

Light travels in straight lines through a transparent substance, if the density of the substance is the same throughout. A single line of light is called a ray, and a number of parallel rays make up a beam. A sunbeam is an example. If a sunbeam enters a dark, dusty room, its straight path is shown by the illuminated dust particles. On a foggy night a street lamp sends out straight lines of light in all directions. These observations show that light travels in straight lines. The same thing is shown by the formation of images and shadows.

168. Images Through Small Openings. An easy way to learn how images are formed is to let light pass through the small opening of a "pin-hole camera." The camera (Fig. 141) consists of a box with both ends removed. A hole is cut in one of the remaining four sides of the box,