If the difference be small, it may be compensated by means of the hand regulator; if large, however, other means must be taken to cause the machines to take up their due proportion of the load. If the series coils of the several dynamos be provided with small adjustable resistances, in the form of German silver or copper ribbon inserted in series with the coils, the distribution of the current in the latter may be altered by varying the resistance attached to the individual coils. The effect of the series coils upon the individual armatures in raising the pressure may be adjusted, and the load thus evenly divided among the machines. Shunt and Compound Dynamos in Parallel.--It is not practicable to run a compound dynamo and a shunt dynamo in parallel, for, unless the field rheostat of the shunt machine be adjusted continually, the compound dynamo will take more than its share of the load. CHAPTER XXXI DYNAMO FAILS TO EXCITE This trouble is of frequent occurrence in both old and new machines. If a dynamo fail to excite, the operator should first see that the brushes are in the proper position and making good contact, and that the external circuit is open if the machine be shunt wound, and closed if series wound. In starting a dynamo it should be remembered that shunt and compound machines require an appreciable time to build up, hence, it is best not to be too hasty in hunting for faults. The principal causes which prevent a dynamo building up are: 1. Brushes not properly adjusted; 2. Defective contacts; 3. Incorrect adjustment of regulators; 4. Speed too low; 5. Insufficient residual magnetism; 6. Open circuits; 7. Short circuits; a. In external circuits; b. In dynamo. 8. Wrong connections; 9. Reversed field magnetism. Brushes not Properly Adjusted.--If the brushes be not in or near their correct positions, the whole of the voltage of the armature will not be utilized, and will probably be insufficient to excite the machine. If in doubt as to the correct positions, the brushes should be rotated by means of the rocker into various points on the commutator, sufficient time being given the machine to excite before moving them into a new position. Defective Contacts.--If the different points of contact of the connections of the machine be not kept thoroughly clean and free from oil, etc., it is probable that enough resistance will be interposed in the path of the exciting current to prevent the machine building or exciting. Each of the contacts should therefore be examined, cleaned, and screwed up tight. Ques. Which of the contacts should receive special attention? Ans. The contact faces of the brushes and surface of the commutator. These are very frequently covered with a slimy coating of oil and dirt, which is quite sufficient to prevent the machine exciting.