[Ill.u.s.tration: _Fig. 17. A MAGNET_]
"Well, is magnetism the same as electricity?"
"Both exhibit the same manifestations. Magnetism is nothing more than a body charged with electricity. The electricity, which appears to travel around the conductor (A), extends out for some distance from its body, and produces what is called a magnetic field. This is the case whether the magnet is a permanent one, like the earth, or whether the conductor is charged by a dynamo."
"What is the difference between the north and the south pole?"
"There is really no difference. The terms north or south and positive or negative are mere relative designations, and are distinguished simply by the movement or direction of the travel of the current. You will remember when we made the battery, it was shown that the current, outside of the battery, moved from the positive to the negative pole.
That was merely stating that it moved from the north to the south pole outside of the earth, and from the south to the north pole inside of the earth. The current is, therefore, from one magnetic pole to the other."
"What I cannot understand is why the magnetic poles should be at the north pole and at the south pole."
"The magnetic poles are not at the poles of the rotation of the earth, but hundreds of miles away, to one side of the poles on which the earth rotates; but they are near enough to the real poles, for all purposes, so that the needle points to what we call the north pole of the earth.
Any magnetized body must have these two opposite poles. If it is a body, like a bar of iron, one end is called north and the other south. Look at this other sketch (Fig. 18) and you will see how the currents flow in the two magnets. In this case the large body (E) represents the earth and the small body (M) the magnet. Now notice that the current going around the large body moves to the right, or to the north pole, whereas the current in the small magnet (M) flows in the opposite direction."
[Ill.u.s.tration: _Fig. 18. MAGNETIC INDUCTION_]
"And does the current flowing around the bars, as you have shown, make the small magnet turn around so that it is always parallel with the large magnet, and make the north pole of one magnet at the same end with the south pole of the other magnet?"
"Yes; to make it still clearer, I make two more drawings (Figs. 19 and 19a), in which two sets of magnets are shown. In the first of these pairs of magnets (Fig. 19), the two north poles approach each other, and the two south poles are opposite each other. The currents, if you will notice, at the north poles move toward each other, and at the south poles move away from each other. They are, therefore, acting against each other, and the result will be that the magnets will move away from each other. If, now, one of the magnets is turned so the poles of one magnet approach the opposite poles of the other magnet, as shown in the second view (Fig. 19a), they will attract each other, because the current is permitted to flow through the two magnets in the same direction without one conflicting with the other."
[Ill.u.s.tration: _Fig. 19. Fig. 19a. THE TWO MAGNETS_]
"Is that the reason it is stated that likes repel and unlikes attract?"
"Yes."
In order to take advantage of this knowledge, knowing that the earth is a great permanent magnet, it was necessary to make a small magnet, and so suspend it that it would turn freely, and the magnetic north and south could then be determined.
To do this the battery which had been previously made was brought into play. George took a hand in the work, and while they were preparing the metal for the little bar, said: "You spoke about a permanent magnet.
What other kinds of magnets are there?"
"Magnets are permanent or temporary. A permanent magnet is one in which the electricity resides, or remains, as it does in the earth. A temporary magnet is one which has magnetism imparted to it only while a current of electricity is pa.s.sing around it."
"How is the current made to pa.s.s around it?"
"By wrapping an insulated wire around it, and sending a current through the wire. When that is done the same thing is done to the bar as the bar of the permanent magnet exhibits. As soon, however, as the current through the wire ceases, the bar is again demagnetized. That is, it ceases to be a magnet."
"We have the small bars ready, Professor. What is the next step?"
"It must be hardened so as to make it a flinty steel. The harder the better, so that it will preserve the magnetism imparted to it."
"Is that the better way to make the temporary magnet?"
"No; in that case the bar should be of the softest iron. Remember, therefore, that for a permanent magnet, use the hardest steel, and for a temporary one, the softest iron."
"Then as we want to make a permanent magnet, must we harden both of the bars?"
"No; for our uses, one must be left soft, because on that we shall want to wind some insulated wire to make a temporary magnet."
The small amount of wire which was on hand was then coated with a thin layer of the ramie fiber, which was carefully wrapped around, so that the different layers of wire could not touch each other. When this was completed, a spool was constructed, which fitted over the little bar or rod, because they were rounded off, and one end of the soft iron rod extended out beyond the spool.
The opposite ends of the winding were then brought out and attached to the terminal wires of the battery. A test showed that the magnet thus made would readily pick up pieces of iron or steel. The Professor then took the hardened steel rod, through which a small hole had been bored, midway between its ends, and laying it down on the table, the projecting end of the temporary magnet which projected from the spool was put into contact with the hard steel rod, and slowly drawn along to the end. The soft bar magnet was then raised up and again repeated, as shown in the drawing (Fig. 20), where the dotted line (A) represents the movement of the end of the temporary magnet.
[Ill.u.s.tration: _Fig. 20. MAKING A PERMANENT MAGNET_]
This was repeated over and over a great many times, and finally the hard steel bar was found to have a charge of magnetism, and for the purpose of providing a means for holding the magnetism, a C-shaped piece of iron was put on the bar, as shown in the detached figure.
"Is that the reason," asked Harry, "why a small piece of metal is always put across the ends of a horseshoe magnet when it is not in use?"
"Yes."
CHAPTER X
STARTING ON THE VOYAGE TO THE WEST
"If I remember correctly, you stated some time ago, Professor, that the barometer indicated the pressure of the atmosphere, and in that way it was useful in letting us know what the weather would be. Before we sail, would it not be well to make one of them? If we had possession of one of the articles, we might not have been caught in the storm the first time we took out No. 3."
"That is a good suggestion. I intended to propose that, because with the barometer and the compa.s.s we shall be equipped with two of the most useful instruments needed."
"I cannot comprehend how the air pressure has anything to do with the weather. Is the air pressure really greater at one time than at another?"
"Heated air ascends, does it not?"
"Yes; I can understand that."
"As it ascends it is, therefore, lighter at that point than normally. On the other hand, moist air is heavier than dry air. These two conditions would be indicated by the barometric column, would they not?"
"I presume they would; but when the air is moisture laden we don"t need a barometer to tell it is going to rain. We know it and feel it. What I particularly wanted to know was how the barometer by its actions would indicate it ahead for any length of time."
"The barometer does not indicate with any degree of accuracy on land; but on sea it has a much better application. The instrument shows the present pressure of the atmosphere, and its variations correspond to atmospherical changes which have already taken place, the effects of which may follow their cause at a greater or less interval."
"Then how could it be ascertained from the instrument when there would be a storm or rain?"
"After a continuance of dry weather, if the barometer begins to fall slowly and steadily, rain will certainly ensue; but if the fine weather has been of long duration, and the mercury may fall for two or three days before any perceptible change takes place; the more time elapses before the rain comes, the longer the wet weather is likely to last."
"Then what indicates dry weather?"
"If, after a great deal of wet weather, with the barometer below its mean height, the mercury begins to rise steadily and slowly, fine weather will come, though two or three wet days may first elapse; and the fine weather will be more permanent in proportion to the length of time that pa.s.ses before the perceptible change takes place."
"Is this the case at all times of the year?"
"The seasons affect the barometer, it is true. A sudden fall of the barometer in the autumn or in the spring indicates wind; in the summer or in hot weather it prognosticates a thunderstorm; in winter, after frost, a sudden fall of the mercury shows a change of wind or a thaw with rain; but in a continued frost a rise of mercury indicates approaching snow."