which incites the alternate currents so formed into one continuous current. The commutator as before stated is fastened on the shaft at the end one, two, three Fig. 13. It consists of three copper plates in the form of a cylinder each segment A´A´A´ covering 115° of the dotted circle Fig. 14. They are screwed to rod CCC and DDD which are insulated by wood and gutta-percha plates EE from the iron mounting E´´ which is in turnescrewed to shaft by set screws shown. The wires one, two, three, have respectively red, white and blue insulation and are put in binding posts DDD marked one, two, three at the factory and if not so placed may work badly. The current enters D goes to B´B´ which there have direct contact with A.

Now in a three-part commutator the spark occurring as the segments pass under the brushes would very quickly destroy the surface and interfere with the currents in the coil. This difficulty is overcome by blowing out the spark by an air blast given at just the right place and time. The manner in which the blast is delivered is as follows: the segments of the commutator

are separated by gaps of about 5° and in front of each of the leading brushes there projects a nozzle, Fig. 15, which discharges an air blast alternately three times in each revolution. The blast itself is supplied by an ingenious piece of mechanism known as the