"There is a compass needle right at your hand waiting to answer that question," I replied. They immediately found that it was a magnet. "Well," I said, "to be really sure that it is a magnet you must find a portion of it that will _repel_ a portion of your compass needle as well as other portions in both horseshoe and needles which attract each other." Whereupon, they found that the portion marked _N_ (Fig. 13) repelled the blue end of the compass needle and attracted strongly the bright end of the needle, while the portion marked _S_ did the reverse. "We will call _N_ and _S_ the poles of the magnet. This is simply a steel bar magnet bent into the shape of a horseshoe." [Illustration: Fig. 13] "You told us," remarked one of the boys, "that steel magnets gradually lose their strength. How then can this be correct as a measuring instrument?" "It is the purpose of the iron case to enable this magnet to retain its magnetism, and if you will examine its field, as we did that of another magnet upon a former occasion, you will find that although this is a strong steel magnet its field does not extend outside of the iron case. It is as though we could box up magnetism and keep it from escaping. "Now if this is like the magneto, where is the armature? The spool-like thing between the poles of the magnet looks just like the armature in one of the magnetos. "Yes, it has an iron core with a coil of insulated wire around it, and you remember that when an electric current is sent around a piece of iron, that iron is made into a magnet, and if it is a magnet it must have poles. It is very delicately poised upon a pivot and will act exactly like your compass needle, which is also a little magnet with poles. I will send an electric current through the wire which surrounds this armature, and you notice that the needle which it carries moves to the right. Notice that the lower end of this armature acts like the blue end of your compass needle in that it is repelled from the pole _N_ of the field and is attracted toward _S_ of the field. In like manner, the upper end or pole of the armature is repelled from _S_ and attracted to _N_ of the field. The blue end of the compass needle is called its north pole because it points north under the magnetic influence of the earth, and so we may call the lower end of the armature its north pole.