When the platinum contacts of the interruptor become worn down after long use, or if for any other reason they require readjustment, they must be set so that the distance between them at the moment that the cam holds them apart is not more than one one-hundredth of an inch. This space may appear to be very small for the work that is to be performed, but it will be sufficient, and a greater distance will result in sparking that will burn down the contacts. In the Eisemann magneto, the adjustable contact is held by two nuts, and small open end wrenches are provided to fit. A wrench should be applied to each of these nuts, and the lower or lock nut loosened. This will free the upper nut by which the adjustment may be made, and the lock nut may then be tightened. The contact faces of the platinums should be kept flat and smooth, and true to each other, so that they come together squarely. If they are pitted after long use they may be faced off with a dead smooth file, but this is a job that requires great care, and must only be done under the most favorable conditions. When completely worn down they should be replaced with new ones.
TROUBLES
Should the engine show a tendency to miss, the first suspicion, if the magneto contacts are known to be correct, should fall on the spark plugs, and the easiest method of testing them out is to replace them with new ones. After considerable use, the spark plug points will burn off, and the size of the gap will increase to such an extent that the spark will find less resistance in traversing the safety spark gap than the gap in the plug.
In order that the ignition may be cut off for the purpose of stopping the engine, a switch is always provided by which the magneto may be short circuited, one pole of the switch being connected to the magneto terminal, or live wire, and the other pole grounded, as shown in Fig. 9. When this switch is closed, the magneto current flows through it in a closed circuit, and as it abandons its path through the interruptor there will be no further action in the coil, the ignition of course ceasing. This switch is often located in the rim or arm of the steering wheel, so arranged that pressing on a button closes the circuit and diverts the magneto current from the coil.
Any accidental short circuit of the magneto current will produce the same effect, and in case of the abrupt cessation of ignition, this is one of the probable causes. A short circuit in the secondary will usually make itself known by the snapping of the sparks as they pass through broken insulation or from a frayed cable end.
The remarks on general magneto troubles and care already made on page [254] also apply to a magneto of this type, and the same rules regarding the protecting of the circuit of the lines of the field must be borne in mind.
H.-T. MAGNETO SYSTEMS
A high-tension magneto differs from the types described in that the armature has a double winding; one, the primary winding, consisting of a few layers of coarse wire, and the secondary winding, placed over the primary, consisting of many layers of very fine wire. As the armature revolves, a current is induced in the primary winding, the circuit of which is kept closed until the armature reaches the position in which the induced current is at its greatest value. An interruptor operated by a cam then opens the primary circuit, and the sudden demagnetization of the armature core that results induces in the secondary winding a current of high pressure that is quite sufficient to jump the gap in the spark plug and to cause ignition of the mixture. The construction of the armature is more delicate than is the case with a low-tension magneto, for the secondary winding must have a large number of layers, and to permit this in the limited space, the wire must be fine. In addition, it must be insulated with the greatest care, for the intensity of the current produced will enable it to seek out and to break down any weakness that exists. There must also be two brushes, one for the primary current and one for the secondary, which complicates the construction.
Fig. 10.—Wiring Diagram, H.-T. Magneto Interchangeable with Secondary Coil.