The secondary current occurs whenever the core is magnetized and demagnetized, and it is clear that the oftener the magnetization occurs the more constant will be its presence. The timer closes and opens the primary circuit, which results in the production of but two impulses of the secondary current, and as these are not sufficient to produce the necessary spark, additional means are employed to make and break the circuit while the timer makes contact. This is accomplished by means of a vibrator, or trembler, which consists of a flat steel spring secured at one end, so that it may vibrate after the manner of a tuning fork. About midway between its ends the point of an adjusting screw touches it, and when the flat spring, or blade, vibrates, it springs away from this screw and returns to it, both screw and blade being tipped with platinum to offset corrosion. If one wire of the circuit is attached to the blade and the other to the screw, the circuit will be complete when the blade touches the screw, and broken when it springs away from it. In the early type of vibrator the blade was set in vibration by mechanical means. At the end of the blade was a weight, which rested on the edge of a cam, the blade then being out of contact with the adjusting screw (Fig. 27). During the revolution of the cam the weight dropped into a deep notch, thus setting the blade into vibration and the circuit being made and broken. This type has been almost entirely superseded by the magnetic vibrator, which is more rapid and accurate. In this the blade is so placed that its free end is close to the end of the core of the coil (Fig. 28). When the timer makes contact and the current passes through the primary winding, the core becomes magnetized and attracts to it the free end of the steel blade, this movement drawing the blade away from the adjusting screw with which it was in contact. The vibrator is so connected into the primary circuit that the current flows from the battery to the adjusting screw, thence to the blade and through the primary winding; when the attraction of the blade by the core draws it away from the adjusting screw, the circuit is broken, and the magnetism then ceasing to exist, the elasticity of the blade causes it to spring back to the screw, to make contact, and to be again attracted by the magnetism.
Fig. 27.—Mechanical Vibrator. When the weight drops into the notch on the cam, the flat spring touches the adjusting screw and the current passes.
Fig. 28.—Magnetic Vibrator. When the magnetism of the core attracts the vibrator the contact is broken at the adjusting screw.
This action continues as long as the timer holds the circuit closed, and a wave of secondary current is set up in the secondary winding for every make and break that results. By means of the adjusting screw, the vibrations may be made long and slow, or short and quick, with a corresponding difference in the secondary current. As the object to be attained is the production of a spark that will ignite the mixture rapidly, the vibrator must be adjusted with this end in view; but it is also advisable to obtain the spark with the least possible expenditure of current, and with the least wear of the parts. The passing of the battery current through the primary winding establishes the condition that exists in the make-and-break ignition system, and the demagnetization of the core will set up an induced current that will result in a spark at the points where the circuit is broken; that is, at the vibrator and timer contacts. This spark can be greatly overcome in the timer by packing it with vaseline, which, being an insulator, breaks down the spark in flowing between the contacts of the stationary and moving parts as they separate. If the vibrator contacts presented no resistance—an impossible condition—there would be no spark, but as they will always be slightly burned and corroded, the spark must be kept as small as the requirements of the secondary current will justify by the adjustment of the screw. In other words, adjust the vibrator so that it gives the best results in the production of the secondary spark with as little sparking as possible at the vibrator contacts.
THE SPARK PLUG
The secondary circuit includes the secondary winding of the coil and the spark plug, which is the device in the combustion space at which the spark occurs. The spark plug provides two points of metal at a fixed distance apart, which are so connected into the circuit that the current must jump from one to the other in order to complete its circuit. The secondary current, as well as the primary, may make use of the metal of the engine as a ground return, and one of the points of the spark plug is therefore in contact with the metal, being supported on a metal sleeve that is screwed into the cylinder. The other point must of course be insulated from it, for if it were not the current could pass from one to the other without jumping the space between them. The insulation is secured by a tube of porcelain or mica set in the sleeve, the lower end of a metal rod that passes through it being the required distance from the wire or projection on the sleeve (Fig. 29). This device must be strong enough to resist the pressure in the combustion space during the compression and power strokes, and must be unaffected by the intense heat.
Fig. 29.—A, Spark plug; B, spark plug in section.