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side of the driving shaft end plate, each holder carrying two brushes so arranged that each brush bears against the slipring in a separate groove. Upon rotation of the armature, the metal segment in one slipring groove makes contact with a brush on one side of the magneto at the same instant that the metal segment in the other slipring groove comes into contact with a brush on the opposite side of the magneto. The marks “1” and “2," appearing
Fig. 105.-Contact Breaker and Distributor Arrangement of Bosch N U 4
High Tension Magneto.
in white on both brush holders, indicate pairs of brushes receiving simultaneous contact, those marked "1" constituting one pair, and those marked "2" the other.
As four-cylinder, four-cycle engines require two sparks per revolution of the crank shaft, and the type "NU4” produces high tension current only every 180° revolution of its armature shaft, the magneto must be operated at engine speed in order to provide the required ignition. It should be taken into consideration that, since
at each interruption of the primary circuit a spark appears at two plugs, the four effective or power sparks required for the four cylinders during every two revolutions of the crankshaft are accompanied by a like number of surplus sparks. Each cylinder receives alternately one effective spark and one surplus spark, the latter occurring exactly 360° behind the former.
In coupling the magneto to the engine, care should be taken that the platinum interrupter screws do not separate too late in their relation to the stroke of the piston. If they do, the surplus spark will occur when the inlet valve is open. With the magneto timed correctly, the extra spark always occurs during the exhaust stroke, when it has no effect on the operation of the engine. The brush holders fit directly into openings in each side of the driviny shaft end plate and are held in place by the “L’-shaped catch springs. These springs are pivoted at one end, and at the other, or rounded end, carry a small boss which, when the spring is in position, rests in a notch in the brush holder and secures it in place. A slight downward pressure and outward pull on the rounded end of the catch spring disengages the spring and permits removal of the brush holder.
To connect the spark plug cables to the magneto, the slipring brush holders are removed and the brushes and brush springs withdrawn. At the base of each of the brush receptacles is a pointed cable fastening screw which is to be withdrawn, and in doing so it is essential to use a narrow-bladed screw driver in order to obviate the possibility of cracking the insulation of the brush holder. The ends of the cables are cut off square and pushed as far as they will go into the cable sockets of the brush holder. The pointed cable fastening screws are then returned to position, piercing the insulation and wires of the cable, thus securing it tightly and at the same time making perfect electrical connections.
Splitdorf Dixie Magnetos.-By adding eight and twelve cylinder models to its line of magnetos, the Splitdorf Co. is able to furnish magneto ignition for any automobile engine now on the market. The Mason principle on which the Dixie magnetos operate is a radical departure from ordinary magneto practice, and possesses many features of great interest. In the first place the rotating
Fig. 106.—Diagram Explaining Action of Splitdorf “Dixie" Magneto
at A. B-How Ignition is Advanced or Retarded. Contact Breaker Construction. D-How Secondary Current is Collected. E-Interior View of Eight Cylinder Distributor. F-Central Member of Distributor for Carrying Brushes.
shaft passes through the magnet poles instead of between them, and instead of carrying an armature on which the windings are placed, this shaft carries two solid polar extensions separated by a non-magnetic distance piece. Surrounding these revolving pole
pieces is a light laminated field structure consisting of two pole pieces F and G, Fig. 106, and a straight core on top. This core carries both primary and secondary windings. The principle of operation is that of sending magnetic lines alternately in opposite directions through the field structure. It will be seen that the pole extensions S and N are simply a means of carrying the magnetic lines from the main magnet to the laminated field structure, and that they do not change their polarity. In the four- and sixcylinder models each polar extension embraces about 90° of the tunnel.
Path of the Flux.—When the pole N is adjacent to G, Fig. 106, left, the magnetic flux flows in the direction of the arrows through the core of the windings from left to right. Continuing the rotation of the poles until they occupy a vertical position it will be seen that the field of the magnet is shorted through the pole pieces, cutting out the magnetic flux entirely from the core. Passing this point in rotation the pole extension N then comes into a position adjacent to F, causing the magnetic lines to flow once more through the core, but this time in the opposite direction, that is, from right to left. This reversal of direction of the magnetic flux is, of course, a necessary feature in any magneto and is the means of inducing the current in the windings.
In order to render this reversal easy and complete, the path for the magnetic lines is made up of thin iron laminations such as are used also in the construction of the armature in the ordinary magneto. The Splitdorf Co., however, make the claim for the Dixie construction that a point of great efficiency is obtained since the bulk of iron in the stationary field structure is so small, its size being governed entirely by magnetic requirements. The windings are remarkably small, being wound on a core of only 0.75 by 0.5 in., Fig. 106, D. The core is held in place by two screws passing through slots in the projecting ends. One end of each of the two windings is earthed. The open end of the high tension winding terminates in a contact plate P, Fig. 106, D, embedded in a rubber block at the side of the windings. The open end of the primary winding passes through a brass tube which leads to the base of the magneto, and so to the contact breaker, Fig. 107.
In dismantling, this wire is the only electrical connection to be. loosened.
The Rocking Field—One of the most important features of the magneto is that the whole of the laminated pole structure, including the windings, can be rocked through several degrees. This rocking is accomplished by turning the timer arm of the circuit
Fig. 107.—Sectional View of Splitdorf “Dixie” Inductor Type Magneto.
breaker in the ordinary way to advance or retard the spark. By means of this positive connection between the field and the circuit breaker it is possible to arrange the instrument to produce the sparks either advanced or retarded at the critical moment when the most magnetic lines are being cut. Hence the magneto has no one point in its spark position when the intensity of the spark is maximum or minimum; it is uniform all the time.