The electric rock-drill is now winning its way into the mines which are ventilated with comparative ease as well as into those which are more difficult to supply with air. It is plain, therefore, that on its merits as a conveyer and storer of power the electric current is preferable to compressed air. The heat that is generated and then dissipated in the compression of any gas for such a purpose represents a very serious loss of power; and it is altogether an insufficient excuse to point to the compensation of coolness being secured from the expansion. Fans driven by electric motors already offer a better solution of the ventilation difficulty, and the advantages on this side are certain to increase rather than to diminish during the next few years.
The electric rock-drill, which can already hold its own with that driven by compressed air, is therefore bound to gain ground in the future. This is a type and indication of what will happen all along the industrial line, the electric current taking the place of the majority of other means adopted for the transmission of power. Even in workshops—where it is important to have a wide distribution of power and each man must be able to turn on a supply of it to his bench at any moment—shafting is being displaced by electric cables for the conveyance of power to numerous small motors.
The loss of power in this system has already been reduced to less than that which occurs with shafting, unless under the most favourable circumstances; and in places where the works are necessarily distributed over a considerable area the advantage is so pronounced that hardly any factories of that kind will be erected ten years hence without resort being had to electricity, and small motors as the means of distributing the requisite supplies of power to the spots where they are needed. It was a significant fact that at the Paris Exposition of 1900 the electric system of distribution was adopted.
In regard to compressed air, however, it seems practically certain that, notwithstanding its inferiority to electric storage of power, it is applicable to so many kinds of small and cheap installations that, on the whole, its area of usefulness, instead of being restricted, will be largely increased in the near future. There will be an advance all along the line; and although electric storage will far outstrip compressed air for the purposes of the large manufacturer, the air reservoir will prove highly useful in isolated situations, and particularly for agricultural work.
For example, as an adjunct to the ordinary rural windmill for pumping water, it will prove much more handy and effective than the system at present in vogue of keeping large tanks on hand for the purpose of ensuring a supply of water during periods of calm weather. Regarding a tank of water elevated above the ground and filled from a well as representing so much stored energy, and also comparing this with an equal bulk of air compressed to about 300 pounds pressure to the square inch, it would be easy to show that—unless the water has been pumped from a very deep well—the power which its elevation indicates must be only a small fraction of that enclosed in the air reservoir.
It will be one great point in favour of compressed air, as a form of stored energy for the special purpose of pumping, that by making a continuous small flow of air take the place of the water at the lowest level in the upward pipe, it is possible to cause it to do the pumping without the intervention of any motor.
One means of effecting this may be simply indicated. The air under pressure is admitted from a very small air pipe and the bubbles, as they rise, fill the hollow of an inverted iron cup rising and falling on a bearing like a hinge. Above and beneath the chamber containing this cup are valves opening upwards and similar to those of an ordinary force or suction pump. The cup must be weighted with adjustable weights so that it will not rise until quite full of air. When that point is reached the stroke is completed, the air having driven upwards a quantity of water of equal bulk with itself, and, as the cup falls again by its own weight, the vacuum caused by the air escaping upwards through the pipe is filled by an inrush of water through the lower valve. The function of the upper valve, at that time, is to keep the water in the pipe from falling when the pressure on the column is removed. The expansive power of the air enables it to do more lifting at the upper than at the lower level, so that a larger diameter of pipe can be used at the former place.
Cheap motors working on the same principle—that is to say through the upward escape of compressed air, gas or vapour filling a cup and operating it by its buoyancy, or turning a wheel in a similar manner—will doubtless be a feature in the machine work of the future; and for motors of this description it is obvious that compressed air will be very useful as the form of power-storage. Excepting under very special conditions, steam is not available for such a purpose, seeing that it condenses long before it has risen any material distance in a column of cold water.
"The present accumulator," remarked Prof. Sylvanus P. Thompson in the year 1881, referring to the Faure storage batteries then in use, "probably bears as much resemblance to the future accumulator as a glass bell-jar used in chemical experiments for holding gas does to the gasometer of a city gasworks, or James Watt's first model steam-engine does to the engines of an Atlantic steamer." When Faure, having in 1880 improved upon the storage battery of Planté, sent his four-cell battery from Paris to Glasgow, carrying in it stored electrical energy, it was found to contain power equal to close upon a million foot-pounds, which is about the work done by a horse-power during the space of half an hour. This battery weighed very nearly 75 lb. It nevertheless represented an immense forward step in the problem of compressing a given quantity of potential power into a small weight of accumulator.
The progress made during less than twenty years to the end of the century may be estimated from the conditions laid down by the Automobile Club of Paris for the competitive test of accumulators applicable to auto-car purposes in 1899. It was stipulated that five cells, weighing in all 244 lb., should give out 120 ampere-hours of electric intensity; and that at the conclusion of the test there should remain a voltage of 1·7 volt per cell.