The Optical Magic Lantern Journal (February 1897)

The Optical Magic Lantern Journal and Photographic Enlarger, 33 behaviour under certain circumstances, of the electricity which he proposes to train to his service. I have already said that the chief source of electricity of the kind that is employed to run an arc light is the dynamo, and that this engine may be likened to a pump whose function it is to force some of the electricity which is supposed to be all-pervading, away from one position where it leaves an emptiness, into another which thus has more than its normal quantity. So far the matter is easily understandable. It does not require any very acute perception to see that when you have too much of anything in one place which is anxious to get across to another where there is too little, and you offer to convey it across the space which it cannot pass without your help, you can justly demand a tol! for so doing. . That is to say, the discontented “something” which is so anxious to transfer itself from the one place to the other, may be made to doa certain amount of work in its passage, and, naturally, that amount of work is proportional to the extent of its anxiety to make the change. For if it is only moderately anxious, and you say that you will give it the means of passing from place to place provided it will do a large amount of work, it will conclude that it is better off where it is, and nothing will come of your offer. Similarly, if the distance which it has to travel be very long, it has all this dificulty—the travelling expenses, as it were-— to take into consideration, and it will only be worth its while to undertake a smaller quantity of work, by way of quid pro quo, than if the distance were shorter. Those are the terms, then, on which you can make electricity do work for you. In return for a free passage in the direction in which it desires to go, it will be willing to perform a certain amount of work in direct proportion to the extent of discontent with its present quarters, which prompts it to make the re:moval, and inversely proportional to the length of the journey. Now, unfortunately, we lanternists who have had no previous dealings with this curious customer that we call electricity, are naturally at a loss to know how to measure the extent of this discontent, or to arrive at a due estimate of the travelling expenses, so how are we to know what amount of work to demand as fair payment? Wecan realise that the “discontent” is a kind of ‘“pressure,” and we can measure the distance with a yard measure, but we do not know how to gauge this pressure, and what is the use of knowing how far it is from one point to another if we have no idea of the difficulties to be overcome in each yard ? The simplest way will be to seek an analogy in some other form of power or energy of which we are conversant, and our yard measure, and quart pot, and Bourdon pressure-gauge may come in useful after all. Hlectricity, we are told, is incompressible, therefore air will not do for our analogue. If we take water we shall be all right upon that point, and perhaps it is the best that we can find, but we shall have to drop it before long, for it will not hold good through all the ramifications of the subject. Let us pre-suppose the existence of a steam engine as the original source of energy, both in the case of water power and electricity, for here is a junction place from which they both may spring, and it will not be necessary to say that the coal is the source of the steam, or that the light and heat of the sun were responsible for the birth of the coal. I know you will accept the spirit of my poor explanations and not examine them so hypercritically as that, or I would never have attempted to set them down. The steam engine works a pump whose duty it is to lift water from a cistern at a low level into another some considerable height above. The amount of water which it will raise in a given time is proportional to its strength divided by the height to which it has to carry the water. The result of the pumping is that we have what we call a ‘ head” of water— “pressure ’”’ will be a better term for this particular purpose—and we therefore have a stored up power to accomplish a certain amount of work, which will be in direct proportion to the extent of this pressure and the amount of water which is stored. The pressure we measure aS SO many pounds on the square inch, and the water can be measured in quarts. This definition and measurement of pressure remains the same if we abolish the upper cistern altogether, and carry the water direct from the pumps to the place where the work is ready for it to accomplish. This change is necessary in order to keep up the analogy to electrical power, for electricity cannot be stored —using the word in its proper acceptation. In the same manner we may take the thickness of the stream of water as it comes from the pump as a means of estimating its