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. [Illustration: 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. [Illustration: Fig. 15]