"You will notice that the binding posts on the bottom of this ammeter are marked, one positive, +, and the other, negative -. The electric current now enters the instrument by the post marked + and after passing around the armature leaves by the post marked -. I will reverse the connections and thus send the current around the armature in the other direction, and you notice that its poles are now reversed. The lower end which was formerly the north pole of the armature has now become the south pole, as proven by the fact that it is repelled from the south pole of the field and attracted to its north pole. This carried the needle to the left, and inasmuch as the zero is in the middle of the scale we may with this instrument both measure the amount of current and tell its direction. You will recall that when we connected the magneto with this instrument, it indicated that the magneto sent the current first in one direction and then in the other, which we call an 'alternating current.' But you notice that the current which I am using in this laboratory flows continuously in one direction. This is called the 'direct current.' We shall find out how a dynamo may produce a direct current at another time. Let us not forget, however, that we have repeated Ampère's discovery, and found out that the direction in which we send the current around an electro-magnet determines which end shall be its north and which its south pole. If you will note carefully which way the wire is wound around the armature you will see that when I send the current in at the positive post it is passing around the north pole of the armature opposite to the direction in which the hands of a clock move. If I reverse the current it passes around the lower end of the armature in the same direction as the hands of a clock move and then this end becomes a south pole. This is 'Ampère's rule,' and it is what candidates for admission to college are very careful to learn.
"Before we replace the face of this ammeter I must call your attention to a wire running by a short cut from one binding post to the other, s ([Fig. 14]). Suppose a represents the wire around the armature. Electricity, like water, goes more readily through a big conductor than a small one and more readily through a short than a long conductor. If s and a were water pipes, each having a stop-cock, we might easily adjust the cocks so that one tenth of the water would go through a and nine tenths through s. Or, indeed, without stop-cocks, the size and length of s and a might be so apportioned that one tenth of the water would flow through a and nine tenths through s. This is precisely the adjustment which has been made with reference to the flow of electricity through this instrument. s is called a 'shunt.' When the shunt is out all the current goes through a and when the shunt is in only one tenth of the current goes through a. I have two other shunts, each of which may be put in the place of s. With the second only one hundredth of the current goes through a and with the third only one thousandth of the current goes through a. Thus I have an instrument which will measure anything from one thousandth of an ampere up to ten amperes.
Fig. 14
"In this laboratory we pay about one cent for an ampere of electricity for one hour. Twice as much coal must be consumed to furnish two amperes as one, and twice as much coal must be consumed to furnish an ampere for two hours as for one hour. Hence we need an instrument which will keep account of time as well as amount of current. Such an instrument we must look into next.
"Just before we pass to that, however, let me ask if you have ever heard of a 'shunt-wound' dynamo. Can you guess from the way we have just used the word 'shunt' what the expression could mean with reference to a dynamo?" Without hesitation the boys told me that it meant that the field and armature were wound parallel to one another, as shown by diagram in [Fig. 15]. In which case the electric current which the machine generates divides, part of it going around the field and part around the armature. Another type, called series-wound dynamos, is indicated by diagram in [Fig. 16], in which case the electric current goes through field and armature in succession. Under either of these circumstances, how can the armature move with reference to the field? The answer will appear in the next chapter.
