Chapter 16
Electrical Firing
[In 2010 - DON'T try anything on this page - some of it sounds
really, really dangerous and would give Health and Safety
something to get their teeth into !]
Not withstanding the introduction of the Martini, the Simms-Bosch, and other magneto methods of firing the charge in the cylinder, the coil and accumulators will probably hold their own for some years to come. Therefore some directions for charging cells may be of use. The reader is probably aware that an accumulator is a secondary battery, a battery that gives no power of itself, unlike a primary battery such as a Daniel’s, Grove’s, or bichromate battery, but is capable of storing a charge of electricity, or, in other words, undergoing a certain chemical change by the action of the current put into it, and giving out a current due to another chemical action when the source of electricity is removed, and the cell or cells coupled with an induction coil or a lamp.
The makers generally send out directions for charging, but however well these may be understood by an electrical expert, they are unintelligible to most men.
On looking at an accumulator it will be noticed that one pole is either marked with a + or painted red this is the positive. It will also be noticed that when two or more cells are coupled the positive of one is connected to the negative of the other.
There are few towns or villages where there is not some house lighted by electricity, and here the cells can be recharged, presuming that the electrical current is a direct one, and not an alternating current. First, it is necessary to find out which is positive wire from the dynamo; to do this a pole finder must be used. A simple way of finding this is to take two strips of sheet lead 3in. or 4in. long and 1/4in. or 1/2in. wide; fix these to a strip of wood, and couple with the dynamo. Immerse these in a small glass jar of dilute sulphuric acid and water, about one acid to ten water. It is necessary to keep a lamp in circuit, otherwise the current passing would be too great for the fuse. In a few minutes one of the plates will be noticed to be changing colour, becoming brown or chocolate. This is the positive. The wire should then be marked +, or, what is better, its covering should be painted red.
It would not do to connect the current from dynamo direct to the accumulators, for the electric motive force or pressure would be too great and damage the cells, if the installation is one of fifty volts, a forty-five volt lamp must be put in circuit. If it is a circuit of one hundred volts, a ninety-five volt lamp, for each accumulator requires an electromotive force, generally written as E M F, of 2.5 volts to charge it; two accumulators five volts, and so on, must be used. In the writer’s house the dynamo is driven by an oil engine, the voltage being fifty. In an adjoining works shop is a forty - five volt lamp, which is placed in circuit with the two accumulators of the motor car. This charges them slowly at about one ampere. This is left charging all the evening. If the cells are nearly exhausted, as after a long tour, a second lamp is placed in circuit, the cells being charged at two amperes. It must be remembered that on a hundred volts circuit each lamp takes only half an ampere, so the charging will be more slowly done, two lamps being required to give one ampere, and, of course, four to give two. Before starting charging, the plugs which seal the cells must be taken out. When the cells gas freely, or have gased for an hour or more, they may he considered charged. Some cells will gas before others, the cause of which is somewhat obscure.
The cells should be kept clean and free from dust or dirt. If acid is spilled from the vent holes it will be absorbed by the dust or dirt, and very likely lead to short circuiting, that is, the electricity will pass from one terminal to the other through the damp dirt, and the charge would be lost. Any white or green crystals due to the action of the acid on the copper or lead must be carefully removed, and the wires and clamps cleaned and covered with vaseline.
Where there is no electric installation cells are sometimes charged by primary batteries. The Daniel cell, and also the Boron battery, have been used for this. At first sight it might be suggested that if a primary battery can do the work, why should it not be used on the car to excite the coil, but the bulk of a primary battery would be at least three or four times the bulk of the accumulator, besides it would be very difficult to prevent spilling the acids by the vibration of the car, for primary batteries cannot very well be sealed as accumulators are, as the acids and chemicals have to be continually renewed.
Accumulators should never be permitted to run down completely, and if the car is to be put away for a month or six weeks the cells should be fully charged first. The coils used on cars are what are known as induction coils, or Rhumkorf coils, so named after the inventor, a Paris optician, who improved on the early coils, and made a coil that would give a fairly long spark, and by which many interesting experiments were made. The only practical use coils were put to for many years was for vaporising minute quantities of metal for spectroscopic investigation; for nearly forty years no other practical use was found for the coil, except in firing the fuse in blasting and firing the gaseous mixture in the Lenoir gas engines - an early gas engine which only had a very ephemeral existence. It is a curious fact that almost simultaneously there have been three useful applications discoveredI for the induction coil, namely, for producing the Röntgen rays, Marconi’s system of wireless telegraphy, and firing the charge in motor cars.
In 1831 Faraday showed at the Royal Institution that an electric spark could be produced from magnetism; he did this by bringing a bar of soft iron surrounded by a coil of wire in rapid contact with a magnet. It was also found that, if a bar of iron were magnetised by a current of electricity passing through a wire coiled round it, on breaking contact with the battery a current would be produced by induction in another coil of wire surrounding that in which the current from the battery was circulating, and on the thickness and length of this wire the quality of the spark depended, for a spark to jump across an air gap of 1/2in. or 3/4in. the secondary must be a long thin wire. The primary, that is, the wire which carries the battery current, is comparatively stout.
Fig. 48 shows diagrammatically a small coil. A is the iron core, composed of a number of soft iron wires. If this core were of solid metal a current would be induced in it, and would considerably weaken the desired current in the secondary or thin wire. This is surrounded by the bobbin B, usually made of ebonite, or sometimes of wood, and well saturated with paraffin wax, which is an excellent insulator; on this is coiled the primary wire of large diameter, usually about 16 W.G. The primary is covered with several layers of paper soaked in paraffin wax, and then the secondary C is wound on, each layer being carefully insulated from the other by several layers of paraffin waxed paper.
To break contact in the primary and produce a spark, an iron hammer D is attracted by the magnetism of the soft iron core. This hammer is on a spring E, and carries at its end a platinum contact piece F, which rests against a similar one on the end of the screw. Now as the battery current passes through E F G directly the hammer is attracted the contact is broken at F, the hammer falls back, and the process is repeated. The screw G is for adjusting the contact points F as they wear away.
If such a coil is working bright sparks will be noticed at F; this may he called self-induction, for it detracts from the spark in the secondary. A condenser is employed to suppress this spark. It consists of a number of sheets of tinfoil, each insulated from its neighbour by sheets of paraffin waxed paper. The even numbers of sheets are connected to each other, and the odd numbers to themselves; and then by wires these two batches of tinfoil are connected one on each side of the contact piece F, that is, one to the base of the spring E, and the other to the standard carrying the screw G. The condenser is shown at H, for it is usually placed in the base. Of course, there are many more sheets than shown in the diagram. The condenser draws away the undesirable spark from the primary, and considerably increases the power of the coil.
The construction of coils is, of course, a totally different branch of trade from motor car making; the coils are made by electricians, and supplied to the makers of the cars.
One wire from the battery is led to some metallic part of the engine; the other wire to one of the primary binding clamps of the coil. From the other primary binding clamp a wire goes to the insulated spring on the contact-making piece. One secondary wire is led to some metallic part of the engine; the other to the binding clamp in the porcelain of the ignition plug. Fig. 49 shows a Benz ignition plug in section. The porcelain stem with the platinum wire passing through it is packed with asbestos yarn; the spark, of course passes between the points of the wire.
In the De Dion motor tricycle the arrangement of coil is different. There is no vibrating spring or trembler worked by the magnetism of the coil, but contact is made and broken by a spring, which falls into a gap in a disc on the second speed shaft.
The first magneto firing gear used in gas engines was made by Martini, one of the inventors of the Martini-Henry rifle. It consisted of a Siemens armature of the old or H type fixed between the poles of a powerful permanent magnet; at the time of firing this is caused to make about a quarter of a revolution on its axis by the action of a powerful spring. This produces an instantaneous current in the wire; to fire the charge the electric circuit is led through the combustion chamber, and at the proper moment contact is mechanically broken therein, a spark being the result, which fires the mixture.
This machine was largely, and no doubt is still, used on the Continent, but it would be too heavy for motor car work. For this purpose the Simms-Bosch was introduced. Instead of making the armature oscillate, or make its quarter turn, the armature is fixed, but a shell of soft iron revolves for one-fourth of a revolution, and induces magnetic action in the armature. A diagram (fig. 52) will explain this. The armature A between the poles of the magnet is fixed; a light iron shell B cuts the magnetic field and induces magnetism in the armature, and therefore an instantaneous current in the wire, as the spring pulls the iron shell over it, by a rod and lever, breaks contact in the cylinder, thus firing the charge. A cam on the second motionshaft of the engine compresses the spring, which produces a sudden movement of the iron shell.
