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.
"The electric current which I am sending through the armature comes first through one ordinary 16-candle-power electric lamp which you see lighted on this 'resistance board,' as it is called, and you notice that the needle points to .5. This means that half an ampere of electricity is passing through this lamp. I will now send the current through a 32-candle-power lamp, and you notice that the needle points to one, indicating that one ampere is required to light that lamp. But what prevents the needle from going farther, and what brings it back to zero each time?" The boys discovered a very small spring, like the hair spring of a watch, coiled around the pivot of the armature. "So, then, one ampere of electricity gives magnetism to this armature so that it may pull against its coiled spring hard enough to carry the needle to the point one. Twice as much electricity will give it magnetism enough to carry it to two, and so on across the scale.
"The full name of this instrument is Ampere meter, which by usage has been shortened to ammeter. It was named in honour of André Marie Ampère, who was born at Lyons, in France, in 1775, the year our Revolutionary War broke out. He died in 1836. When Oersted made his famous discovery of the action of an electric current upon a magnetic needle, in 1819, Ampère was in middle life (forty-four), and took up the same line of research with great vigour. The next year, 1820, he discovered what you will doubtless enjoy rediscovering now.