Recall that all dynamos are generators of alternating current. If the brushes rest upon rings upon the axle they send forth alternating current--but if the brushes rest upon commutator bars they send forth direct current. Now we will have two sets of brushes, one pair of which shall rest upon the rings on the axle, and they will collect alternating current for the cottage, while the other pair will slide over the commutator bars and collect direct current for the machine shop. I have constructed a model which will make it plain. Here is a piece of a broom handle (Fig. 138), one foot long, which shall represent the axle of an armature. _a b c d_ is a stout wire which represents the coil of the armature. In this case it has no iron at its centre. Nevertheless it will serve as an armature having one loop of its coil left. _e_ and _f_ are rings, sawed from a piece of brass pipe, which fit snugly upon the axle. Another ring of the brass pipe was sawed lengthwise, as shown in Fig. 139. These two halves are also fastened upon the axle and one end of the wire loop, _c_, is fastened to one of these, and the other end of the loop, _b_, is fastened to the other half of the ring. These two halves of the piece of brass pipe are placed so that their edges are near to each other but do not touch on either side of the axle. The two ends of this wire loop are also connected with the rings _e_ and _f_. A short wire connects _b_ and _e_ and another connects _c_ and _f_ passing through the wood of the axle, as shown by the dotted line. We will now revolve this loop slowly about its axle in a strong magnetic field. To produce this field I will send two amperes of electricity through the coils of wire (Fig. 140), which surround two iron pole pieces that are screwed into an iron base. Between the poles _N_ and _S_ of this electro-magnet we will thrust this wire loop and revolve it as an armature very slowly. Meanwhile I connect two wires to my sensitive ammeter and let their free ends brush along on the rings _e_ and _f_. The needle of the ammeter swings to and fro for each half revolution of the armature, showing an alternating current of .01 amperes. If this armature had many turns of wire instead of this one loop, if it had an iron core, and if it should revolve at high speed, the results would differ in degree but not in kind. [Illustration: Fig. 140]