The correct pressure is attained when the brushes collect the full current without sparking, while their pressure upon the commutator is just sufficient to overcome ordinary vibration due to the rotation of the commutator. [Illustration: Figs. 667 to 669.--Method of winding cables with marlin. When connecting the feeders and dynamo and service leads to a switchboard, the wires are often _served_ with marlin. By serving is meant to tightly wrap the wires of each set together with marlin. A tool for serving may be made as in fig. 667, using a piece of oak 2 ins. wide, 7/8 in. thick and 14 ins. long, having four holes drilled through it, as shown. The marlin is passed through the holes, commencing at the hole nearest the handle, the object being to cause a strain on the marlin at the point where it passes around the wire, so that the marlin may be wrapped tightly. It is necessary to serve the first four or five inches by hand, pushing the winding into the conduit as far as possible. This acts as an additional protection to the wires where they leave the conduit. The serving is continued, as in fig. 668, to within four or five inches of the first lug by means of the serving tool, passing the ball of marlin around the wires with the serving tool. The wires are then bent in shape, as in fig. 669. To serve the wires properly it is necessary to tie the ends of the wires taut. The wires should be straightened and run together so as to be parallel, being bound with tape at different points to keep them so. When the serving is complete the marlin should be thoroughly painted with a moisture resisting compound. The marlin serving will stiffen the wires and they can be bent very neatly to avoid touching the bus bars of the board. When painted the marlin hardens so that it is difficult to bend the wires after the paint has dried. It then requires a strong pressure to bend them. The marlin acts as an additional insulation and mechanical protection to the wires, and while no harm would result from the wires coming in contact with the bars while thus protected, it looks better to bend them so as to avoid touching the bars.] Direction of Rotation.--This is sometimes a matter of doubt and often results in considerable trouble. As a general rule, a dynamo is intended to run in a certain direction; either right handed or left handed according to whether the armature, when looked at from the pulley end, revolves with or against the direction of the hands of a clock. Dynamos are usually designed to run right handed, but the manufacturers will make them left handed if so desired.