The records of the chronograph can be made in two ways: one by a pen on a piece of paper tape, and the other by a scratch on a piece of smoked paper.
So by that means the progress of the "force" of the explosion can be measured. It is necessary also to time the movement of the "heat" of the explosion, for the two may not travel together, and the difference between them may let in some light as to the nature and behaviour of the explosion. So for this second purpose a second set of circuit-breakers are used. Each of these consists of a strip of thin tinfoil stretched across the gallery. Being placed edgeways to the moving current of gas, the force of the explosion has no effect upon it, but the heat instantly melts it. Normally, current flows through the strip, and so the melting is signalised by the cessation of the current, which event is recorded by the chronograph.
Thus the speeds at which the force and the heat of the explosion travel are ascertained. Another important fact which needs to be found is the amount of the force, or the pressure, at different points. For this purpose pressure-gauges can be connected to the gallery at the desired spots by means of flexible tubes. This flexible tube is necessary in order that the vibration of the steel shell, due to the explosion, shall not be communicated to the instrument. The pressure, finding its way along the flexible pipe, raises a piston against the force of a spring, and the distance to which it is raised forms, of course, a measure of the pressure inside the gallery at the point to which the tube is connected. The pressure is recorded by the action of the piston in moving a style which just touches against the surface of a moving paper. There are three styles in all marking this paper. The first is the one just mentioned. The second is held down on to the paper by an electro-magnet energised by current flowing through a fine wire stretched across the gallery just where the explosion originates. This fine wire is broken at the moment of the explosion, whereby the current is cut off and the style raised. It therefore makes its mark until the moment the explosion occurs, and then leaves off. The end of that line, therefore, shows the time of the explosion. Meanwhile the first style is drawing a straight line, but as soon as the pressure begins to be felt by the pressure recorder this style moves and the line slopes upward. Upward it goes as the pressure increases, until it has reached its height, after which it descends, until the style is drawing a straight line once more. Thus the rise and fall of the line represents the rise and fall of the force of the explosion.
Then comes the matter of time. How soon after the explosion occurred did the pressure begin to be felt? How long did it take to reach its maximum and how long to die out again? These questions need answers which the apparatus so far described does not give. True, the speed of the paper may be known approximately, but all that I have described will occur within the space of a fraction of a second, and it is difficult to tell the speed of the paper with sufficient accuracy. Therein we see the purpose of the third style. It is attached electrically to the "tenth-of-a-second time-marker." This consists of a weight suspended at a height. The force of the explosion lets it drop. The moment it starts to fall it causes the style to make a mark on the paper. When it has fallen a certain distance the style makes another mark. And the distance that the weight falls between the making of the two marks is so adjusted that the space between them on the chart represents exactly a tenth of a second. Thus a scale is formed upon the chart by which the other times can be measured. There is the line terminating at the moment of explosion; the straight line changing into an up-and-down curve, representing the time and the variation of the pressure; finally there are the two marks representing a tenth of a second by which the other marks recorded upon the chart can be interpreted.
But the mere pressure and velocity of the explosion form but a part of the knowledge desired. How the explosion is formed, whether or not the coal-dust is burnt up entirely, whether, indeed, it be the dust itself which burns or coal-gas given off by the dust under the heat of the preliminary explosion, what the gas is which is left by the explosion at various stages—these are important things to be known, and they can only be ascertained by taking samples of the gases in the gallery at different moments during and after the explosion. To obtain these samples bottles are used, but the question is how to get them filled at just the right time. Into the shell of the gallery holes are drilled, and to these the metal bottles or flasks are screwed, a pipe leading from the mouth of each bottle well in towards the centre of the gallery. The end of this tube is closed by a cap of glass above which there stands poised a little hammer. Controlling the hammer is an electrical device called a "contact-maker," so arranged that just at the desired moment the hammer falls, breaking the glass, and admitting a sample of the gas in the gallery, the bottle and its tube having previously had the air exhausted from them, so that on the glass being broken the gas is sucked in.
At the same moment a weight falls, attached to the end of a cord, and this, on reaching the end of its tether, closes the end of the tube, and the sample is imprisoned until such time as the bottle can be disconnected and taken away to the laboratory for its contents to be analysed.
The contact-makers are of two kinds. In one the pressure of the explosion raises a piston which completes a circuit allowing current to flow through the very fine wire which prevents the fall of the hammer. This fine wire being fused by the current, the hammer falls and does its work. The other kind, which are used when the force of the explosion is not enough to raise a piston, is operated by one of the tinfoil circuit-breakers. A magnet, being energised by current passing through the foil, holds up a curved bar over two cups of mercury. Broken by the heat of the explosion, the foil cuts off this current, de-energises the magnet, and allows the bar to fall with its ends in the mercury. This completes another circuit, permitting current to pass to the fine wire, whereby the hammer is released. By connecting a bottle to a contact-maker at a distance the sample can be obtained at any desired period of the explosion. If, for instance, the sample is to represent the immediate products of combustion, it is placed near to the contact-maker. Then the sample is drawn in practically at the moment of explosion. If, on the other hand, it is the after-damp that is to be sampled, then the bottle would be connected to a contact-maker a long way from the seat of the explosion, with the result that its glass cap would not be broken until some considerable time had elapsed after the explosion has passed the bottle. The time also during which the bottle is drawing in its sample can be adjusted by varying the length of the cord to which the weight is attached.
And last of all must be mentioned the employment of a kinematograph, capable of taking twenty-two photographs per second, for observing the effects at the ends of the gallery (see illustrations).
Thus records are obtained of the force and heat of the explosion, its mechanical and thermal effects upon the walls of the gallery, or, if it were in a real pit, the effects which it would have in shaking and in heating the workings, and the men labouring in them. This and the analysis of the gases producing and produced by the explosion, derived from the contents of the bottles, give sound data upon which can be built up reliable theories as to the nature of colliery explosions and the way to prevent them, results which could be obtained in no other way. No one can help being struck with the thoroughness and ingenuity of the means adopted to these ends, and it is no exaggeration to say that it is a splendid example of thoroughly scientific methods applied to an important industrial investigation. It will be interesting to conclude this account with a brief mention of some of the results to which these painstaking efforts have led.
First in importance the fact is placed beyond doubt that coal-dust, which in bulk will only burn slowly, will, when well mixed with air, explode. And no combustible gas need be present to aid in the explosion.