Bad Condition of Commutator.--If the surface of the commutator be rough, worn into grooves, or eccentric, or if there be one or more segments loose or set irregularly, the brushes will be thrown into vibration, and sparking will result. A simple examination of the commutator will readily detect these defects. A rough and uneven commutator is due to bad adjustment of brushes, bad construction of commutator, and to neglect generally. If allowed to continue, it results in heavy sparking at the brushes, and the eventful destruction of the commutator. The fault may be remedied by filing or re-turning the commutator. [Illustration: Fig. 716.--Rough and grooved commutator due to improper brush adjustment and failure to keep brushes in proper condition.] Ques. How is an untrue commutator detected? Ans. If the commutator be untrue, the fact will be indicated when the machine is slowed down by a visible eccentricity, or by holding the hand, or a stick in the case of a high tension machine, against the surface while revolving, when any irregularity or eccentricity will be apparent by the vibration or movement of the stick. The only remedy for an untrue commutator is to re-turn it in the lathe. Ques. What should be done in case of high segments? Ans. They should be gently tapped down with a mallet, and if possible the clamping cones at the commutator end should be tightened. If it be impossible to hammer the segments down, they should be filed down to the same diameter as the rest of the commutator, or the commutator re-turned. For low segments, the only remedy is to pull out the segments, or turn commutator down to their level. Ques. Explain the term "flats on the commutator." Ans. This is the name given to a peculiar fault which develops on one or more segments of the commutator. It is not confined to dynamos of bad design or construction, but frequently appears on those of the highest class, and may be recognized as a "pitting" or "flattening" of one or more segments. Ques. What is the effect of flats on the commutator? Ans. Sparking at the brushes. Ques. What are the causes which produce flats? Ans. Periodical jumping of the brushes due to a bad state of the commutator, bad joint in the driving belt, a flaw, or a difference in the composition of the metal of the particular bar upon which it appears. But more frequently flats may be traced to a more or less developed fault, such as a break, either partial or complete, in the armature coil. The break may occur either in the coil itself, or at the point where its ends make connection with the lug of the commutator, or at the point where the lug is soldered to the segment. Ques. What should be done in case of flats? Ans. The brushes should be examined to see if any periodical vibration take place. If such be the case, the cause should be removed, the flat carefully filed or turned out, and the brushes readjusted.