A dynamo to run properly must be kept clean and dry. The parts that require the greatest care are the commutators and brushes. The commutator should be kept clean by wiping it with a hard cotton cloth. The occasional application of a little vaseline tends to diminish friction between the commutator and brushes. Oil should never be used for this purpose. As the commutator becomes roughened with age, it should be smoothed by holding fine sandpaper against it while the machine is revolving. If the commutator gets out of true (out of adjustment), it must be turned down in a lathe. If it becomes wet, the insulation of the armature and field coils will be injured or destroyed, because in such a case resistance between the frame and the electrical part becomes low. A commutator in which this trouble occurs is said to be "badly grounded." There should be an insulation resistance of 10,000,000 ohms. The bearings of a dynamo require no more attention than the bearings of any other machinery.