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.
CHAPTER XXVIII
ELECTRO-CULTURE
About thirty years ago a Swedish scientist, Professor Lemström, travelled extensively in the Polar regions, and he was greatly struck by the development of the Polar vegetation. In spite of the lack of good soil, heat, and light, he observed that this vegetation came to maturity quicker than that of regions having much more favourable climates, and that the colours of the flowers were remarkably fresh and clear, and their perfumes exceptionally strong. This was a surprising state of things, and Lemström naturally sought a clue to the mystery. He knew that peculiar electrical conditions prevailed in these high latitudes, as was shown by the wonderful displays of the Aurora Borealis, and he came to the conclusion that the development of the vegetation was due to small currents of electricity continually passing backwards and forwards between the atmosphere and the Earth. On his return to civilization Lemström at once began a series of experiments to determine the effect of electricity upon the growth of plants, and he succeeded in proving beyond all doubt that plants grown under electrical influence flourished more abundantly than those grown in the ordinary way. Lemström’s experiments have been continued by other investigators, and striking and conclusive results have been obtained.
The air surrounding the Earth is always charged to some extent with electricity, which in fine weather is usually positive, but changes to negative on the approach of wet weather. This electricity is always leaking away to the earth more or less rapidly, and on its way it passes through the tissues of the vegetation. An exceedingly slow but constant discharge therefore is probably taking place in the tissues of all plants. Experiments appear to indicate that the upper part of a growing plant is negative, and the lower part positive, and at any rate it is certain that the leaves of a plant give off negative electricity. In dull weather this discharge is at its minimum, but under the influence of bright sunshine it goes on with full vigour. It is not known exactly how this discharge affects the plant, but apparently it assists its development in some way, and there is no doubt that when the discharge is at its maximum the flow of sap is most vigorous. Possibly the electricity helps the plant to assimilate its food, by making this more readily soluble.
This being so, a plant requires a regular daily supply of uninterrupted sunshine in order to arrive at its highest possible state of maturity. In our notoriously variable climate there are many days with only short intermittent periods of bright sunshine, and many other days without any sunshine at all. Now if, on these dull days, we can perform at least a part of the work of the sunshine, and strengthen to some extent the minute currents passing through the tissues of a plant, the development of this plant should be accelerated, and this is found to be the case. Under electrical influence plants not only arrive at maturity quicker, but also in most cases their yield is larger and of finer quality.
Lemström used a large influence machine as the source of electricity in his experiments in electro-culture. Such machines are very suitable for experimental work on a small scale, and much valuable work has been done with them by Professor Priestly and others; but they have the great drawback of being uncertain in working. They are quite satisfactory so long as the atmosphere remains dry, but in damp weather they are often very erratic, and may require hours of patient labour to coax them to start. For this reason an induction coil is more suitable for continuous work on an extensive scale.
The most satisfactory apparatus for electro-culture is that used in the Lodge-Newman method, designed by Sir Oliver Lodge and his son, working in conjunction with Mr. Newman. This consists of a large induction coil supplied with current from a dynamo driven by a small engine, or from the public mains if available. This coil is fitted with a spark gap, and the high-tension current goes through four or five vacuum valve globes, the invention of Sir Oliver Lodge, which permit the current to pass through them in one direction only. This is necessary because, as we saw in [Chapter VIII]., two opposite currents are induced in the secondary winding of the coil, one at the make and the other at the break of the primary circuit. Although the condenser fitted in the base of the coil suppresses to a great extent the current induced on making the circuit, still the current from the coil is not quite uni-directional, but it is made so by the vacuum rectifying valves. These are arranged to pass only the positive current, and this current is led to overhead wires out in the field to be electrified. Lemström used wires at a height of 18 inches from the ground, but these were very much in the way, and in the Lodge-Newman system the main wires are carried on large porcelain insulators fixed at the top of poles at a height of about 15 feet. This arrangement allows carting and all other agricultural operations to be carried on as usual. The poles are set round the field, about one to the acre, and from these main wires finer ones are carried across the field. These wires are placed about 30 feet apart, so that the whole field is covered by a network of wires. The electricity supplied to the wires is at a pressure of about 100,000 volts, and this is constantly being discharged into the air above the plants. It then passes through the plants, and so reaches the earth. This system may be applied also to plants growing in greenhouses, but owing to the confined space, and to the amount of metal about, in the shape of hot-water pipes and wires for supporting plants such as vines and cucumbers, it is difficult to make satisfactory arrangements to produce the discharge.