The Elements of Electrical Ignition.
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A Brief Explanation of Standard Systems.
We promised in our issue of. the 30th May last (page 318) that the general principles which govern all ignition systems, with explanations such that readers who possess but little electrical knowledge should readily understand them, might be expected at an early date in this journal. It will not be possible to give descriptions of the particular systems of particular makers, and our references will be confined to those details which are to be found, in varying form, in all the modifications of the three systems generally known as : (a) high-tension, jump-spark, with coil and battery • (b) low-tension, wipe-spark, with magneto; and (c) high-tension, jump-spark, with magneto. The first-mentioned system is the one that has been in most general use until quite recently, and, as an understanding of the principles involved in its constitution and application will help the reader more readily to comprehend the details of the other two systems, we shall deal with it first.
High-tension System, with Coil and Battery.
The essential details of the coil and battery system are : a coil; a battery; a switch; a mechanical means of making contact at the precise instant that a closed circuit is required; a sparking plug; and the necessary wires to make all the connections. The coil is an instrument, by means of which an induced or secondary current is produced in the secondary windings, such induced current having so high an electromotive force that a gap made in the high-tension circuit will not interfere with the flow of the current, which will jump across the gap in the form of a spark. It is this spark of which use is made to fire the charge of gas in the cylinder. Referring to Figure 1, it will be seen that the coil consists of : a bundle of soft-iron wires which form a core (L), around which are wound the primary (E) and the secondary (M) windings of the coil ; a contact-breaker or trembler ; and a condenser. The whole of these parts are fitted into a neat case of suitable size, in order that the delicate portions may be protected. The primary or lowtension winding consists of a few turns of insulated copp2r wire of relatively large diameter, one end of which is attached to the terminal (F), whilst the other end of the wire is secured to the bridge-piece (D). Closely wound over the primary winding is the high-tension or secondary winding, which consists of a large number of turns of very fine, silkcovered, copper wire, each layer of which is most carefully insulated by separating sheets of paraffined paper, and also by the varnishing, with shellac varnish, of each layer of wire. The type of battery used in connection with the coil is not a matter which affects the ultimate results, but any most complete tyro in electrical matters will realise that the battery is capable of giving out a current of electricity, if it is kept properly charged. We would, however, advise the van owner and user to employ only the best that can be purchased, as inferior batteries are more often than not a source of trouble and annoyance. Of the switch, little need be said, except that it should be waterproof and well insulated. The " commutator," as the mechanicallyoperated, rotary, switch which closes the primary or battery circuit is erroneously called, makes contact once n each revolution for a single-cylinder engine, and two, three, four, or six times for engines having a corresponding number of cylinders. It consists of a drum, mounted on, bul insulated from, the shaft (R), which rotates at the same speed as the engine camshaft, i.e., half the r.pon. of the crankshaft. The circuit is closed when the segment (j) passes under the " brush " (G), which "brush" is fixed to, but insulated from, the carrier (H). It will be noticed that the segment is in metallic contact with the shaft, and that the carrier is free to swing round its centre, in order that the precise instant of the actual contact between G and j may be made earlier or later in relation to the position of the piston within the working cylinder : when the carrier is moved round in the same direction in which the shaft is rotating, the time of contact is retarded, and, when it is moved in a direction contrary to that of the shaft, the timing is advanced. These operations cause the spark to occur late or early as the case may be.
The action of the coil is as follows :—The current from the battery passes through the primary winding (E), by way of the terminal (F), the bridge-piece (D), the adjusting-screw (C), the trembler-blade, and the terminal (A), from whence it passes through the switch and the " commutator "to the shaft (K), the circuit to the battery being completed through the metal of the engine. The immediate result of this current is that the soft-iron core (L) is converted into a powerful electro-magnet and so attracts the soft-iron armature (B) which is affixed to the trembler-blade. When this blade is drawn down by the magnetic action, the platinum contacts on the blade and the adjusting-screw (C) are no longer in electrical contact, the current ceases, the core loses its magnetism, and the armature is released, thus allowing the blade to spring back and bring the points into contact again, when the same operation is repeated with lightning rapidity, so long as the brush (G) and the segment ( J) remain in contact. Each time the current is interrupted, a secondary current is " induced " in the windings (M) : this induced current has a very high electromotive-force, or " pressure," by reason of the large number of turns round the primary winding, and is conducted to the sparking plug, by means of a highly-insulated copper wire, from the terminal (N); it returns to the other end of the winding by way of the metal of the engine, or " earth," and the terminal (A) to the point of connection (0) with the primary wire.
When a current is passing through a wire, there is a field of force, or a radius of influence, around it, as may easily he demonstrated by the simple experiment shown in Figure 2. A fairly stout wire is passed through a hole in a stiff card, and the ends of the wire are connected to the terminals of a battery ; while the current is passing through the wire, sprinkle some iron filings on the card, and it will be noticed that they will arrange themselves in regular circles around the wire, thus proving that there is some influence being brought to bear upon them by the passing current. If another wire is placed parallel to the first one, and within the field of force, a current will be momentarily induced in it. This is precisely comparable to what we have in the coil under discussion. The induced current is only momentary, and at the instant of the circuit being closed, as when the platinum contacts touch each other, the direction of the induced current is the opposite to that of the Primary or battery current ; a second momentary current is induced at the instant the platinum contacts are separated, as when the armature (B) is attracted to the core, but this current flows in the same direction as the primary current.
A simple experiment to demonstrate the presence of a "field."
A. brief consideration of the foregoing facts will pave the way to an understanding of another very important part of the coil, namely the condenser. If, When a wire is placed parallel to another wire, through which is passing a current, a current is induced in the first wire, it will be obvious that such induction is continually taking Place in the adjacent coils of the primary winding, as well as in the secondary winding. This phenomenon is known as " self-induction." The current which is induced at the instant of completing the circuit flows in an opposite direction to that of the original current, and is in opposition to the latter ; it must, therefore, tend to weaken the original current, but, when the circuit is broken, it flows in the same direction as the primary, and, therefore, the latter is strengthened. Now this " extra " current only comes into existence when the circuit is broken, hence, when a break . is made and the current arrives at the platinum contacts of the trembler and the adjusting-screw, the current proceeds to jump the gap which exisis. If such a proceeding were allowed, the resulting spark would be very brilliant, but the action of the coil would be very sluggish, and the high-tension spark would be useless for exploding the charge in the cylinder. The condenser is employed to prevent this extra current from passing as a spark across the platinum contacts, and to use it for the purpose of making the coil more efficient.
The condenser consists of alternate sheets of tin-foil and waxed paper, and the odd numbers of the tin-foil sheets are allowed to overhang the waxed paper at one end, whilst the even numbers of the tin-foil are allowed to overlap the paper at the other end. These layers form two distinct elements, as in a Leyden jar, and are capable of receiving a charge of electricity, The =denser is connected up as a " shunt" across the bridge-piece (D) and the trembler-blade. at the points P and R, and receives the late or " extra " current, instead of letting it pass in the form of a spark. The surfaces of the elements of the condenser are so large, that they offer a ready vent for the extra-current, which, like water, follows the path of least resistance and " spreads Oat " over the tin-foil sheets, charging them to a high potential. The condenser being connected as a shunt across the primary, it is incapable of retaining the charge, which is, therefore, discharged in an opposite direction, through the primary windings, and is used for the purpose of neutra.lising any residual magnetism that may be present in the iron core. (L).