A-B-C of Electricity

by Wm. H. Meadowcroft

Fig. 19

All we have to do, then, is to collect this electricity from the two ends of the wire, and use it. If we should attach two wires to the two ends of this wire on the spool, they would be broken off when it turned around, so we must use some other method. We fix on the end of the spool (which is called an "armature") two pieces of copper, so that they will not touch each other (as in Fig. 19), and fasten the ends of the wire to these pieces of copper. This is called a "commutator," and, as you see, is really the ends of the wire on the spool. Now we get two thin, flat pieces of copper and fix them so that they will rest upon the copper bars of the commutator, but will not go round with it. These two flat pieces of copper are called the "brushes," and they will collect from the commutator the electricity which is gathered in the wire around the spool. As the brushes stand still, two wires can be fastened to them, and thus the ampères of current of electricity, acted upon by the volts pressure, can be carried away to be used in the lamps, for you must remember that as long as the spool turns around it gathers more electricity while there is any magnetism for the wire on the spool to pass through. The constant revolving of the spool creates so much electricity that it is driven out from the wire on the spool, through the commutator to the brushes, and there it finds a path to travel away from the pressure of the new electricity which is all the time being made.

In this way we get a continuous current of electricity in the two wires leading from the commutator, and can use it to light electric lamps or for other useful purposes.

In explaining this to you, so far, we have used as an illustration of the magnet one of the steel permanent magnets in order to make the explanation more simple, but now that you understand how the electricity is made, we must explain to you something about the magnets that are used in dynamo-machines. We can perhaps make this more clear by giving another example.

Suppose you had a dynamo which was lighting up 100 of the incandescent lamps, each of 200 ohms resistance and each requiring 100 volts pressure. Now each lamp would take just a certain quantity of electricity, say half an ampère; so, the 100 lamps would require one hundred times that quantity. But, if you turned off 50 of these lamps at once, the tendency would be for the pressure to rise above the 100 volts required for the other 50, and they would be apt to burn out quicker. It is plainly to be seen, then, that we must have some means of regulating the magnetism so as to regulate the lines of force for the wire on the armature to cut through. We can do this with an electromagnet, but not with a permanent magnet, because we cannot easily regulate the amount of magnetism which a permanent magnet will give.

There is another reason why we cannot use permanent magnets in a dynamo, and that is because they cannot be made to give as much magnetism as an electromagnet will give.

Thus you will see that there are very good reasons for using electromagnets in making dynamo-machines. Let us see now how these electromagnets and dynamos are made, and then examine the methods which are followed to operate and use them.

You must remember, to begin with, that in referring to wire used on magnets and armatures and for carrying the electricity away to the lamps, we always mean wire that is covered or insulated. In electric lighting, insulated wire is always used, except at the points where it is connected with, the dynamo, the lamps, a switch, or any point where we make what is called a "connection."

As the shape of the magnets is different in the dynamos of various inventors, we will take for illustration the one that is nearest the shape of the horseshoe and the shape that is generally used in illustrating the principle of the dynamo. This is the form used by Mr. Edison, whom we have previously mentioned. This form is shown in Fig. 20.

Now, although this magnet appears to be in one piece, it really consists of five parts screwed together so as to make, practically, one piece. The names of the parts are as follows: F, F are the "cores"; C the "yoke," which binds them together; and P, P the "pole pieces," where the magnetism is the strongest. These pole pieces are rounded out to receive the armature, which, as you will remember, is the part that turns around.