Electricity passes through some things more easily than it does through others. Those that it passes through easily are said to be good conductors of electricity. There are some things that let so very little pass through or over them that they are called non-conductors. Such are glass and silk. The different metals, copper, silver, iron, etc., are good conductors.
The supports of lightning-rods and telegraph wires.
You have seen how a lightning-rod is fastened to a house. It rests against pieces of wood. Observe what the object of this is. Iron lets the electricity or lightning pass much more easily than the wood does. Now, if the rod was fastened to the house by iron supports, the lightning, as it came down the rod, might go into the house by some of these supports, instead of going down by the rod into the ground.
The iron is called a good conductor, while the wood is a poor conductor. Glass is a poorer conductor still. It is so poor a conductor that it is called a non-conductor, as I have before told you. It is for this reason that the telegraph wires are fastened to glass knobs on the posts. The object is to have all the electricity go along on the wires, and not let any of it escape down the posts. If a very little of it should escape down each post, by the time it came to the end of the journey there might not be enough left to do any good.
Dr. Franklin experimenting with his kite.
Silk, I have told you, is one of the non-conductors. Dr. Franklin made use of silk in the experiment by which he discovered that lightning and electricity are the same thing. He managed in this way: He made his kite of a large silk handkerchief instead of paper. He had on it a pointed iron wire, and the string of the kite was fastened to this wire. This kite he sent up in a thunder-storm, when there was a plenty of electricity in the clouds.The iron wire would of course receive some of the electricity, and it would not go from the wire to the kite, because that was made of silk, which, you know, is a non-conductor. It would go down the string, this being tied to the wire. Passing down the string, it would go to Dr. Franklin’s hand, and down his body into the earth. It would do this silently, because it would keep going a little at a time all the while. But he managed to prevent the electricity from coming to his hand. He stopped it on the way. He did this by tying a silk ribbon to the hemp string, and holding the kite by this ribbon, as you see in the picture. The electricity could not go through this silk, and so it staid in the hemp string.
How Dr. Franklin drew the lightning down from the clouds.
Dr. Franklin now fastened a key to the end of the hemp string. A great deal of the electricity now passed to the key, because the metal of which the key was made was so good a conductor. It was a much better conductor than the string, and so the electricity, as we may say, spread all over it. It was a real receiver of the electricity, like the brass receiver of the electrical machine. Accordingly, when Franklin put his knuckle near the key, he received a shock from it, just as one does from the knob of the brass receiver. After a little time it began to rain, and then the shocks were harder. The reason was, that the string, when wet, was a better conductor than when dry, and so the electricity came on it faster to the key.
In this way Dr. Franklin drew the lightning down from the clouds in so small a quantity that he could find out what it was. He found that it was just the same as the electricity that wemake by the electrical machine, and he could bottle it up in the same way that we do the electricity from the brass receiver. This he could do by holding the Leyden jar with its brass knob to the key. The electricity would go into it just as it does from the receiver when we are working the machine.
What Franklin proved.
Before Franklin tried this experiment with his kite it was supposed that the lightning was electricity, but it was only supposition. No one knew that it was so. It was never proved till Franklin sent up his silk kite to find out about it. It was supposed that lightning was electricity simply because the effects of lightning were similar to the effects of the electricity of the machine when a great deal of this electricity was made. Experiments were tried which showed that the machine electricity, when there was enough of it, tore things to pieces, and killed animals, just as lightning does; but the difficulty was that no one had ever seen what a little of the lightning would do. This Franklin found out by bringing some of it down out of the clouds by the string of his kite, and bottling it up for use in the Leyden jar. Before his experiments nothing was known about lightning except as it was seen in large quantities going from cloud to cloud, or coming down to the earth and shivering a tree, or plowing up the ground, or perhaps killing some animal or some man. Nothing was known of it in a small way until Franklin showed us so much about it by his experiments.
Suggested the use of lightning-rods.
Lightning-rods protect in two ways.
It was these experiments of Dr. Franklin that suggested the use of lightning-rods. These rods protect houses in two ways. One way is this: If the lightning comes down directly toward ahouse in a considerable quantity, instead of striking the house, it will go down the rod into the ground. Another way in which the rod affords protection is this: Sometimes the lightning or electricity goes down the rod from the clouds above in a continual stream of very small quantity, just as it went down the string of Franklin’s kite. A cloud with a great deal of electricity in it often has it discharged in this quiet way.
Use of the points on them.
You know that there are points on the ends of lightning-rods. These are to receive the electricity. It will go to them better than it would to a blunt rod. We know that this is so in working the electrical machine described on page 145. Instead of having simply the blunt end of the receiver near the rubber, there are points on that end of it to receive the electricity as fast as it is made.
Questions.—What things are called good conductors of electricity? What are called non-conductors? Why are lightning-rods supported against a building by pieces of wood? Why are telegraph wires fastened to glass knobs on the posts? How did Franklin make his kite? Why did he make it of silk instead of paper? How did he prevent the electricity that came down the string from going through him into the ground? Why was the key so good a receiver of electricity? Tell about his taking shocks from it. Why were the shocks stronger after it began to rain? How did he bottle up the electricity that he thus drew from the clouds? Why was it supposed before his experiment that electricity and lightning were the same thing? Why was it not known to be so? In what two ways do lightning-rods protect houses? Why are lightning-rods pointed?
Questions.—What things are called good conductors of electricity? What are called non-conductors? Why are lightning-rods supported against a building by pieces of wood? Why are telegraph wires fastened to glass knobs on the posts? How did Franklin make his kite? Why did he make it of silk instead of paper? How did he prevent the electricity that came down the string from going through him into the ground? Why was the key so good a receiver of electricity? Tell about his taking shocks from it. Why were the shocks stronger after it began to rain? How did he bottle up the electricity that he thus drew from the clouds? Why was it supposed before his experiment that electricity and lightning were the same thing? Why was it not known to be so? In what two ways do lightning-rods protect houses? Why are lightning-rods pointed?