To avoid this, a condenser is used, which is built into the magneto. This condenser consists of two sets of tinfoil sheets, sheets of opposite sets alternating with each other, and being separated by sheets of insulating material. All the sheets of each set are metallically connected, and one set is connected with the primary winding, while the other set is grounded. These condensers arc capable of absorbing an electrical charge, and their capacity is so proportioned that they will take up the entire charge of extra current produced when the contact points of the circuit breaker separate - that is, the extra current instead of appearing in the form of a spark across the gap, passes into the condenser.
The magneto armature is positively driven from the motor crankshaft and the current impulse in the armature always occurs when the piston is in a certain position. Since in regular operation of the motor the charge is ignited just an instant before the completion of the compression stroke, the magneto armature is so set relative to the engine crankshaft that the maximum induction effect occurs at this moment. This construction is termed a fixed spark. However, quite a few users demand a variable spark; or in other words, to vary the time of the cycle when ignition occurs. In order to make this possible, the circuit-breaker housing F is so arranged that it can be rocked around its axis, being provided with a lever arm for this purpose, which is connected with the spark lever on the steering gear.
In the Mea magneto, owing to its construction, the circuit breaker and the magnets are moved around their axis so that the armature will hold its relation to the pole pieces, whether advanced or retarded.
One model of Eiseman magneto incorporates an automatic spark advance, which is accomplished by the action of centrifugal force on a pair of weights attached to one end of a sleeve, through which runs the shaft of the magneto, and hinged at the other end of the armature. Along the armature shaft run two helicoidal ridges, which engage with similarly shaped splines in the sleeve. When the armature is rotated, the weights begin to spread and exert a longitudinal pull on the sleeve, which in turn changes the position of the armature with reference to the pole pieces. In this way the moment of the greatest induction is advanced or retarded, and with it the breaker in the primary circuit.
The high-tension current is distributed to the spark plugs in the following manner: One end of the secondary winding is grounded through the primary winding, since it is attached to one end of the primary, the other end of which is grounded. The other end of the secondary generally leads to the collector ring, from which the current is taken off by a carbon brush. From here the current is carried through a spring contact conductor to a distributor on the magneto. This distributor consists of an insulated disc, in which are imbedded on the inner side one central contact piece and four or six sector-shaped contact pieces, the numl>er depending upon the number of cylinders on the engine. This distributor also comprises a shaft which carries a gear wheel meshing with another on the armature shaft. The reduction between these gears is two to one, so that the armature shaft makes two turns. The large gear wheel carries a brush holder, containing a carbon brush, which makes contact simultaneously with the central contact piece and one of the sector-shaped contact pieces, which are connected by means of wiring or cable to the spark plugs.
A magneto must be so designed that it will give a sufficiently hot spark at a comparatively low engine speed. This ability implies the ability of generating very large and hot sparks and enormously high tensions at high engine speeds.
The electromotive force generated in the secondary winding is limited to the size of the spark gap of the spark plugs: for. as soon as the tension reaches a point sufficient to jump this gap, the discharge occurs and there is no further increase in the electromotive force. If, by chance, the gap between the spark plug electrodes become large enough that there is no chance for the sparks to pass in the ordinary way, the electromotive force in the secondary winding might build up to such an extent as to puncture the insulation of the winding and ruin the armature. To avoid this, a safety gap is provided in which the gap is larger than in the spark plugs. Under ordinary conditions, no spark will pass between the two terminals of the safety gap. However, should the condition mentioned above arise, a discharge will occur at the safety gap, preventing the electromotive force from rising still higher.