The petrol motor, which to-day is busily engaged all over the world in driving thousands upon thousands of self-propelled vehicles or automobiles, belongs to the important class of internal-combustion engines. Combustion means the operation of burning, and an internal-combustion engine is one in which the motive power is produced by the combustion of a highly explosive mixture of gases. In the ordinary petrol motor this mixture consists of petrol and air, and it is made by means of a device called a “carburetter.” By suction, a quantity of petrol is forced through a jet with a very fine nozzle, so that it is reduced to an extremely fine spray. A certain proportion of air is allowed to enter, and the mixture passes into the cylinder. Here it is compressed by the rising piston so that it becomes more and more heated, and at the right point it is ignited. Combustion takes place with such rapidity that it takes the form of an explosion, and the energy produced in this way drives forward the piston, which turns the crank-shaft and so communicates motion to the driving-wheels.
The part played by electricity in this process is confined to the ignition of the compressed charge of petrol and air. This may be done in two ways; by means of an accumulator and a small induction coil, or by means of a dynamo driven by the engine. At one time the first method was employed exclusively, but to-day it is used as a rule only for starting the car engine, the second or magneto method being used when the engine has started up.
In accumulator ignition the low-tension current from the accumulator passes through an induction coil, and is thus transformed to high-tension current. This current goes through a sparking plug, which is fixed in the head of the cylinder. The sparking plug contains two metal points separated by a tiny air gap of from about 1/30 to 1/50 inch. This gap provides the only possible path for the high-tension current, so that the latter leaps across it in the form of a spark. The spark is arranged to take place when the piston is at the top of its stroke, that is, when the explosive mixture is at its maximum compression, and the heat of the spark ignites the mixture, the resulting explosion forcing down the piston with great power. In practice it is found better as a rule to cause the spark to pass very slightly before the piston reaches the extreme limit of its stroke. The reason of this is that the process of igniting and exploding the charge occupies an appreciable, though of course exceedingly small amount of time. Immediately on reaching the top of its stroke the piston begins to descend again, and if the spark and the top of the stroke coincide in time the explosion does not take place until the piston has moved some little distance down the cylinder, and so a certain amount of power is lost. By having the spark a little in advance of the piston, the explosion occurs at the instant when the piston begins to return, and so the full force of the explosion is utilized.
In magneto ignition the current is supplied by a small dynamo. This generates alternating current, and it is driven by the car engine. The current is at first at low pressure, and it has to be transformed to high-tension current in order to produce the spark. There are two methods of effecting this transformation. One is by turning the armature of the dynamo into a sort of induction coil, by giving it two separate windings, primary and secondary; so that the dynamo delivers high-tension current directly. The other method is to send the low-tension current through one or more transformer coils, just as in accumulator ignition. Accumulators can give current only for a certain limited period, and they are liable consequently to run down at inconvenient times and places. They also have the defect of undergoing a slight leakage of current even when they are not in use. Magneto ignition has neither of these drawbacks, and on account of its superior reliability it has come into universal use.
In the working of quarries and mines of various kinds, and also in large engineering undertakings, blasting plays a prominent part. Under all conditions blasting is a more or less dangerous business, and it has been the cause of very many serious accidents to the men engaged in carrying it out. Many of these accidents are due to the carelessness resulting from long familiarity with the work, but apart from this the danger lies principally in uncertainty in exploding the charge. Sometimes the explosion occurs sooner than expected, so that the men have not time to get away to a safe distance. Still more deadly is the delayed explosion. After making the necessary arrangements the men retire out of danger, and await the explosion. This does not take place at the expected time, and after waiting a little longer the men conclude that the ignition has failed, and return to put matters right. Then the explosion takes place, and the men are killed instantly or at least seriously injured. Although it is impossible to avoid altogether dangers of this nature, the risk can be reduced to the minimum by igniting the explosives by electricity.
Electrical shot firing may be carried out in different ways, according to circumstances. The current is supplied either by a dynamo or by a battery, and the firing is controlled from a switchboard placed at a safe distance from the point at which the charge is to be exploded, the connexions being made by long insulated wires. The actual ignition is effected by a hot spark, as in automobile ignition, or by an electric detonator or fuse. Explosives such as dynamite cannot be fired by simple ignition, but require to be detonated. This is effected by a detonator consisting of a small cup-shaped tube, made of ebonite or other similar material. The wires conveying the current project into this tube, and are connected by a short piece of very fine wire having a high resistance. Round this wire is packed a small quantity of gun-cotton, and beyond, in a sort of continuation of the tube, is placed an extremely explosive substance called “fulminate of mercury,” the whole arrangement being surrounded by the dynamite to be fired. When all is ready the man at the switchboard manipulates a switch, and the current passes to the detonator and forces its way through the resistance of the thin connecting wire. This wire becomes sufficiently hot to ignite the gun-cotton, and so explode the fulminate of mercury. The explosion is so violent that the dynamite charge is detonated, and the required blasting carried out. Gunpowder and similar explosives do not need to be detonated, and so a simple fuse is used. Electric fuses are much the same as detonators, except that the tube contains gunpowder instead of fulminate of mercury, this powder being ignited through an electrically heated wire in the same way. These electrical methods do away with the uncertainty of the slow-burning fuses formerly employed, which never could be relied upon with confidence.
Enormous quantities of explosives are now used in blasting on a large scale, where many tons of hard rock have to be removed. One of the most striking blasting feats was the blowing up of Flood Island, better known as Hell Gate. This was a rocky islet, about 9 acres in extent, situated in the East River, New York. It was a continual menace to shipping, and after many fine vessels had been wrecked upon it the authorities decided that it should be removed. The rock was bored and drilled in all directions, the work taking more than a year to complete; and over 126 tons of explosives were filled into the borings. The exploding was carried out by electricity, and the mighty force generated shattered nearly 300,000 cubic yards of solid rock.