From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Article from 'The Autocar' magazine explaining the operation of magnetos, with illustrations of Simms, Eisemann, and Bosch models.
| Identifier | ExFiles\Box 4\5\ 05-page041 | |
| Date | 9th November 1912 | |
| THE AUTOCAR, November 9th, 1912. How the Magneto Works. 'hands out' the current from the central disc (brought thereto by the conductor) to the four brass segments in turn. One of these segments is connected up to each cylinder, and the edge of the cover is marked 1, 2, 3, 4, denoting the cylinders. The brass segments on the inside of the cover are connected to the projecting terminals on the outside of the cover, enabling these segments to be connected up by the high tension wires to the four sparking plugs. 'Well, I suppose when the contact breaker platinum points separate the distributer brush must be on one of the brass segments, and the current passes out from the armature to the slip ring along the conductor through the distributer brush to one of the segments?' 'That is quite right. By the time that the armature has rotated through half a revolution to enable the contact breaker to operate again, the distributer brush has moved a quarter of a revolution; then it passes to the next segment, the next cylinder fires, and so on. I will explain this more clearly in connection with the Eisemann magneto, as the segments in this case are not attached to the cover, and the operation of the brush in relation to the contact breaker may be more easily seen. When the cover is removed you will see (figs. 18, 19, 20, and 21) four brass segments, and in front of these lies a black vulcanite arm which, when the armature revolves, rotates over the segments. If you pull this arm out endwise, and look through the hole into which it is projected, you will see the end of the conductor (fig. 18). The gear wheel in this case is hidden behind the vulcanite base. At the back of the arm you will see a square carbon brush, and this rotates over the segments. The arm is on the end of a tubular vulcanite shaft, and the conductor passes up this, so supplying the current to the carbon brush. As the carbon brush moves over the segments the current is imparted to each in turn (figs. 18, 19, 20, and 21). You will notice, also, that when the carbon brush is on a segment the contact breaker points are separated. In the case of the Eisemann the high tension wires going to the sparking plugs are attached to the special terminals, which push into sockets at the top of the brown vulcanite base plate. Each socket is connected up to one of the segments. 'Why is the centre of the carbon brush of the Simms distributer painted white?' 'On the distributer cover you will see four holes which are covered with mica. This white centre can be seen through these holes, so that the position of the distributer can be ascertained without having to take the cover off. In the case of the Simms, the cover is fixed by three screws, and to save the trouble of undoing these, these windows are formed in the cover plate. The construction of the Bosch distributer is practically the same as the Simms, but the cover plate can be removed in a second by pinching together the spring clips and then drawing away the cover (fig. 22). 'Does one have to examine the distributer often?' 'No, very rarely. The carbon brushes and the segments have to be kept clean and dry, but there is very little chance of their getting dirty, as the necessity for inspection is very rare. You must take care when replacing the cover plate to see you put it on straight, otherwise the carbon brush might be damaged. To guard against any trouble it might be well to carry a spare distributer brush, but I do not think you will require it. The segments and distributer plate should be cleaned from time to time with brass polishing paste.' 'How do you switch off the magneto?' 'On the cover of the Eisemann contact breaker you will see a terminal, and this is connected up by a wire to the switch. If you remove the contact breaker cover you will see a little spring box with a carbon centre, which rubs on the central contact breaker screw (fig. 23). When you switch off, the... Image Captions and Labels: Fig. 16.—Three types of magnetos. From left to right, Simms, Eisemann, and Bosch. Labels for Fig. 16: SWITCH TERMINAL, SCREWS SECURING DISTRIBUTER COVER, DISTRIBUTER, TERMINALS, CONTACT BREAKER, MAGNETS, SPRING CLIP. Fig. 17.—Simms magneto with distributer cover removed and placed at the side, showing the segments and control contact. Labels for Fig. 17: CARBON BRUSH, SAFETY SPARK GAP, BRASS SEGMENTS, DISTRIBUTER DISC, DISTRIBUTER GEAR WHEEL, DISTRIBUTER COVER, CONTACT BREAKER, SLIP RING, SEGMENT. | ||