Screw.
What is a screw?
A screw is another form of the inclined plane.
Where is the inclined plane on a screw?
It begins at the bottom of the screw, and winds around and around the screw until it reaches the top, as is shown in [Fig. 60].
Fig. 60.
What is this inclined plane called?
It is called the thread of the screw.
How do the threads affect the screw?
When the threads are far apart, the inclined plane is steep; but it becomes less steep the nearer the threads are together.
When will the screw exert the most power?
The screw will exert the most power, when its threads are nearest together.
Of what use are screws, such as is shown in [Fig. 60]?
Screws are used to fasten hinges and bolts on doors and shutters, and to fasten pieces of wood together in the construction of houses, boats, wagons, and other things.
For what other purposes is the screw used?
The screw is used where much power is needed, as in the vise; where pressure is desired, as in copying letters, and in coining money; and, also, for squeezing the juice from apples, grapes, and sugar-cane.
Fig. 61.
In what does a screw like that in [Fig. 61] work?
It works in a nut which has threads cut on the inside, so that the threads of the screw move in them.
Where is the power applied?
The power is applied to the lever.
Can heavy bodies be raised by means of this screw?
Yes; loaded cars, and even heavy buildings, may be raised by means of a screw of this kind.
CHAPTER XI.
Electricity.
When a cat’s back is rubbed in the dark, what does it give off?
It gives off small sparks.
What are these small sparks called?
They are called electric sparks, or electricity.
If a horse be rubbed in the winter when it is dark, what will be given off from him?
Electricity will be given off from the horse.
When sealing-wax is rubbed with a silk handkerchief, or a piece of flannel, what is produced?
Electricity is produced in the sealing-wax.
What will this electricity in the sealing-wax do?
It will attract bits of paper, straw, feathers, and hairs, when it is held near them.
What other substance will act the same as the sealing-wax?
A dry glass tube will act in the same manner when rubbed.
Where is electricity found?
Electricity is found in every body, and in every thing.
How is electricity developed?
Electricity is developed by motion, by friction, and by almost every change that takes place.
Is electricity found in the air?
Yes; the air is almost constantly electrified.
With what is electricity often accompanied?
Electricity is often accompanied by heat and light.
How many kinds of electricity are there?
There are two kinds of electricity.
What are they called?
They are called positive and negative electricity.
What do these two kinds of electricity always do?
They always attract each other.
When one body contains positive and the other body negative electricity, what will the two bodies do?
They will rush together.
When both bodies are positive or both are negative, what will they do?
They will repel each other.
When a body permits electricity to pass freely through it, what is it called?
It is called a good conductor of electricity.
What substances are good conductors of electricity?
Metals, charcoal, water, animals, vegetables, flame, and smoke.
When a body does not permit the electricity to pass through it, what is it called?
It is called a non-conductor of electricity.
What substances are non-conductors of electricity?
Gutta-percha, sulphur, glass, silk, wool, hair, feathers, cotton, and paper.
What is lightning?
Lightning is electricity passing from one place to another.
When does lightning pass from one cloud to another cloud?
Whenever one cloud becomes filled with more electricity than another, the cloud having the most electricity gives off a portion to the cloud having the least electricity.
When does lightning pass from the earth to a cloud?
Whenever the earth contains more electricity than the cloud does.
When does lightning pass from a cloud to the earth?
Whenever the cloud contains more electricity than the earth does.
How near are the clouds when the lightning comes to the earth?
The clouds are seldom half a mile away when the lightning comes from them to the earth.
What noise is made by the lightning?
The lightning passes so quickly through the air, that it makes a noise called thunder.
Do we always hear the thunder when there is lightning?
No; the lightning is sometimes so far away that the thunder cannot be heard.
What bodies attract the lightning to the earth?
Tall trees, steeples, high houses, and mountains.
Does the lightning do any harm?
Yes; it does much harm by destroying trees, houses, animals, and, indeed, almost everything in its course.
What does lightning sometimes do with buildings?
It sets the buildings on fire so that they are burned.
How are buildings protected from injury by lightning?
They are protected by means of the lightning-rod, which was invented by Dr. Franklin.
Of what is the lightning-rod made?
The lightning-rod is made of some metal, usually of iron or copper, which is a good conductor of electricity.
What does the lightning-rod do when the building is struck?
The lightning-rod conducts the electricity to the earth, so that it does not injure the building.
Must the lightning-rod be higher than the building?
Yes; because the lightning is apt to strike the highest object.
Of what should the point of the rod be made?
The point should be made of silver or platinum, so that it will always remain bright.
Should the rod extend deep into the ground?
The rod should extend four or five feet beneath the surface of the earth.
Why should ashes or charcoal be put around the bottom of the rod?
Because they are good conductors and will help to conduct the lightning into the earth.
How large should the lightning-rod be?
If made of iron, the lightning-rod should be about three-fourths of an inch in diameter.
Why is glass put around the rod where it is joined to the building?
The glass is a non-conductor, and prevents the lightning from leaving the rod.
Why is it not safe to stand near a fire during a thunder-storm?
Because the fire is a good conductor, and sometimes draws the electricity into the room.
What part of the room is the safest place?
The middle of a room is thought to be the safest place.
Is a feather-bed a safe place?
Yes; because the feathers are a non-conductor, and will not attract the lightning.
Is dry air a non-conductor of electricity?
Yes; dry air is a non-conductor of electricity.
When are thunder-storms most frequent?
Thunder-storms are more frequent in summer than in winter, and more so in the afternoon than in the morning.
By what are thunder-storms usually attended?
They are usually attended by some change in the direction of the wind.
Where do thunder-storms generally prevail?
They generally prevail in the lower regions of the air, near the earth.
What is the rapidity of lightning?
The rapidity of lightning is probably not less than 250,000 miles in a second, and the flash does not exceed the millionth part of a second in duration.