Ans. The remaining coils are liable to heat up to a greater extent than formerly, owing to the increased current, hence it is advisable to proceed cautiously in starting the dynamo, since the temperature may exceed a safe limit. If this occur, a resistance may be put in circuit with the field coils, or the speed of the dynamo reduced. [Illustration: Fig. 696.--Fort Wayne pedestal type commutator truing device. When this device is used, the armature is revolved in its own bearings by means of a handle clamped to the pulley. The tool has a horizontal travel of 21 ins., (being 3 ins. wide inside the fastening bolt in the base), and a vertical adjustment of 12 ins., adapting it to machines with commutators up to 36 ins. in diameter.] [Illustration: Fig. 697.--Fort Wayne yoke type commutator truing device for machines having brush mechanism mounted on a yoke carried by the field frame. It consists of a carriage for the tool holder having a screw feed and a bracket for attaching to the brush yoke. The bracket replaces two brush holder brackets on the brush yoke, and is made to fit the yoke of the particular machine on which it is to be used.] Ques. What kind of dynamo is affected by breaks in the external circuit? Ans. A series dynamo. Ques. Name the kind of break that is difficult to locate. Ans. A partial break. Short Circuits.--In a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow. A shunt machine will not excite under these circumstances, for the reason that practically the whole of the current generated in the armature passes direct to the external circuit, and the difference of potential between the shunt terminals is practically nil. Ques. What should be done if it be suspected that the failure to excite arises from this cause? Ans. The main leads should be taken out of the dynamo terminals, then, if due to this cause, the machine will excite. Ques. What parts of a dynamo are specially liable to be short circuited? Ans. The terminals, brush holders, commutator, armature coils and field coils. Ques. How are the terminals liable to be short circuited? Ans. The terminals of the various circuits of the machine are liable to be short circuited, either through metallic dust bridging across the insulation, or through the terminals making direct contact with the frame of the machine. The various terminals should be examined, and if the fault cannot be located by inspection, they should each be disconnected from their circuits and tested with a battery and galvanometer arranged as in fig. 694. Ques. What precaution should be taken with the brush holders? Ans. Since, they are liable to be short circuited through the rocker by metallic dust lodging in the insulating washers, they should be kept clean. Ques. How are the brush holders tested?