. Farm machinery and farm motors. poles, hence the current set upthrough the system is the,sum of all the currents set upat each pole. As the coils of the armature pass acrossthe points midway between the poles, the direction ofcurrent is alternated. The number of alternations to theminute is found by multiplying the number of poles bythe number of revolutions to the minute. ELECTRICAL MACHINERY 479 640. Direct-current dynamo.—For a great many pur-poses it is desirable to have a direct current, that is, onewhich always flows in one direction the same as a cur-rent from a cell. To do this some
. Farm machinery and farm motors. poles, hence the current set upthrough the system is the,sum of all the currents set upat each pole. As the coils of the armature pass acrossthe points midway between the poles, the direction ofcurrent is alternated. The number of alternations to theminute is found by multiplying the number of poles bythe number of revolutions to the minute. ELECTRICAL MACHINERY 479 640. Direct-current dynamo.—For a great many pur-poses it is desirable to have a direct current, that is, onewhich always flows in one direction the same as a cur-rent from a cell. To do this some device must be appliedto the dynamo just at the point where the current leavesthe armature and before it reaches the external device as used in a direct-current dynamo is knownas a commutator. Conumitators are practically split rings secured to, butinsulated from, the shaft of the armature. They take theplace of the accumulating rings of the alternator. Eachpart of the commutator is insulated from the other FIG. 360 FIG. 361 Principle of the coiiiinittator.—Fig. 360 shows a simplecommutator connected to a coil which represents anarmature. A and B are the segments of the metal ring,each of which is connected to the armature. As the arma-ture rotates in the direction indicated by the arrow thecurrent passes off through the side C, out over the ex-ternal circuit through the segment .4, and in throughthe segment B and side D. When the side D has passedinto the i)osition of side C, the current goes out over thecircuit in a similar manner. The brushes E and F must 480 FARM MOTORS be set so they close contact with each side respec-tively and make contact with the other side at the instantthe current in the armature changes direction. 641. Ring armature, direct-current dynamo.—A ringarmature may be made for a direct-current dynamo bywinding on the iron ring a series of coils, the ending ofeach coil being connected to tlie beginning of the junction of
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