Page:Motors and motor-driving (1902).djvu/192

160 chambers by small rocking arms, called the palettes, of which the inside arms are alternately pressed by springs against the surface of the ignition plugs or 'inflammateurs,' and then removed by the action of the cams, the vertical stalks from which lift and hold lifted the outside arms of the palettes.

The action is as follows:—In each cylinder during nearly two engine revolutions—that is, from the moment of its firing until the moment of its next compression approaches—the cam is holding up the outside arm of the palette, and therefore keeping the inside contact broken. As the piston rises during compression, the cam stalk descends and contact is made inside, but the spark of course does not pass until it is again broken. The cam stalk continues to descend until its top is a certain distance (2 mm.) below the outer arm of the palette. It then rises again and strikes the outer arm sharply, breaking contact within, at the identical moment that the magneto being at its 'tight point,' is prepared to give off most current. This occurs in each cylinder in rotation, and as only one contact and break are made at a time, it is evident that one wire only from the magneto is required, and is clamped to the outer ends of each of the four insulated plugs or inflammateurs, with the usual earth return through the frame.

The magneto and contact-breakers, being both thus 'timed' together, through the medium of the cams and gear wheel on the cam shaft, it is in practice only necessary to observe two important points: First, the magneto pinion has a definite position in gear with the cam shaft gear wheel, and should it at any time be necessary to disconnect the magneto, the teeth should be marked. If this is not done it takes a little trouble to find the timing again, but a few turns of the starting-handle trying the teeth in different positions of gear, soon discovers the best position. Secondly, it is important that the distance that the cam stalks descend below the outside arms of the palettes should be maintained at about 2 mm. (the thickness of two halfpennies). A small difference in this distance will not much affect the magneto spark, but it does affect the