Ans. The lead in all good dynamos is very small, and varies with the load and class of machine. The best lead to give to the brushes can in all cases be found by rotating the rocker and brushes in either direction to the right or left of the neutral plane until sparking commences, increasing with the movement. The position midway between these two points is the correct position for the brushes, for at this position the least sparking occurs, and it is at this position that the brushes should be fixed by clamping the rocker. [Illustration: Fig. 680.--Method of taking temperature. In taking the temperature of a hot part, it is convenient to use a thermometer in which the scale of degrees has been etched on the stem. Bind this to the heated part, having first taken the precaution to cover the bulb with waste to prevent the radiation of heat and take the reading when the column of mercury has ceased to rise. The question which most often presents itself to the attendant is how hot can the various parts of a dynamo or motor become and yet be within the safe limit. The degree of heat can be determined by applying the hand to the various parts. If the heat be bearable it is entirely harmless, but if the heat become unbearable to the hand for more than a few seconds, the safety limit has been reached and the machine should be stopped and the fault located. Of course when the solder begins to melt at the commutator connections and shellac begins to "fry out" of the armature and an odor of burnt cotton begins to pervade the air, the safe limit has been far exceeded, and in most cases, as a matter of fact serious damage is the result. To be more definite, _no part of the dynamo or motor should be allowed to rise in temperature more than 80 degrees F. above the temperature of the surrounding air_, excepting in the case of commutators where no solder has been used to connect the leads. These can be allowed to rise to a still higher temperature.] Ques. How does the lead vary in the different types of dynamo? Ans. In series dynamos giving a constant current, the brushes require practically no lead. In shunt and compound dynamos the lead varies with the load, and therefore the brushes must be rotated in the direction of rotation of the armature with an increase of load, and in the opposite direction with a decrease of load. In cases where the dynamos are subjected to a rapidly varying or fluctuating load, it is of course not possible to constantly shift the brushes as the load varies, therefore the brushes should be fixed in the positions where the least sparking occurs at the moment of adjustment. If at any time violent sparking occur, which cannot be reduced or suppressed by varying the position of the brushes by rotating the rocker, the machine should be shut down at once, otherwise the commutator and brushes are liable to be destroyed, or the armature burnt up. This especially refers to high tension machines.