Perpetual Motion

by Percy Verance; Henry Dircks

The Jurin Device

The device which we have designated "The Jurin Device," was not, in fact, invented by Jurin. James Jurin furnished an account of the invention to The Royal Society of London, and it appears in the reports of that society published in 1720. The invention was by a friend of Jurin's whose name he does not give in the account.

Jurin's account of his friend's invention is as follows:

Some days ago a method was proposed to me by an ingenious friend for making a perpetual motion, which seemed so plausible, and indeed so easily demonstrable from an observation of the late Mr. Hawksbee, said to be grounded upon experiment, that though I am far from having any opinion of attempts of this nature, yet, I confess, I could not see why it should not succeed. Upon trial indeed I found myself disappointed. But as searches after things impossible in themselves are frequently observed to produce other discoveries, unexpected by the Inventor; so this Proposal has given occasion not only to rectify some mistakes into which we had been led, by that ingenious and useful member of the Royal Society above named, but likewise to detect the real principle, by which water is raised and suspended in capillary tubes, above the level.

My friend's proposal was as follows:

Fig. 1. Let A B C be a capillary siphon, composed of two legs A B, B C, unequal both in length and diameter; whose longer and narrower leg A B having its orifice A immersed in water, the water will rise above the level, till it fills the whole tube A B, and will then continue suspended. If the wider and shorter leg B C, be in like manner immersed, the water will only rise to same height as F C, less than the entire height of the tube B C.

This siphon being filled with water and the orifice A sunk below the surface of the water D E, my friend reasons thus:

Since the two columns of water A B and F C, by the supposition, will be suspended by some power acting within the tubes they are contained in, they cannot determine the water to move one way, or the other. But the column B F, having nothing to support it, must descend, and cause the water to run out at C. Then the pressure of the atmosphere driving the water upward through the orifice A, to supply the vacuity, which would otherwise be left in the upper part of the tube B C, this must necessarily produce a perpetual motion, since the water runs into the same vessel, out of which it rises. But the fallacy of this reasoning appears upon making the experiment.

Exp. 1. For the water, instead of running out at the orifice C rises upwards towards F, and running all out of the leg B C, remains suspended in the other leg to the height A B.

Exp. 2. The same thing succeeds upon taking the siphon out of the water, into which its lower orifice A had been immersed, the water then falling in drops out of the orifice A, and standing at last at the height A B. But in making these two experiments it is necessary that A G the difference of the legs exceed F C, otherwise the water will not run either way.

Exp. 3. Upon inverting the siphon full of water, it continues without motion either way.

The reason of all which will plainly appear, when we come to discover the principle, by which the water is suspended in capillary tubes.

Mr. Hawksbee's observation is as follows:

Fig. 2. Let A B F C be a capillary siphon, into which the water will rise above the level to the height C F, and let B A be the depth of the orifice of its longer leg below the surface of the water D E. Then the siphon being filled with water, if B A be not greater than C F, the water will not run out at A, but will remain suspended.

This seems indeed very plausible at first sight. For since the column of water F C will be suspended by some power within the tube, why should not the column B A, being equal to, or less than the former, continue suspended by the same power.

Exp. 4. In fact, if the orifice C be lifted up out of the water D E, the water in the tube will continue suspended, unless B A exceed F C.

Exp. 5. But when C is never so little immersed in the water immediately the water in the tube runs out in drops at the orifice A, though the length A B be considerably less than the height C F.

Mr. Hawksbee, in his book of Experiments, has advanced another observation, namely, that the shorter leg of a capillary siphon, as A B F C, must be immersed in the water to the depth F C, which is equal to the height of the column, that would be suspended in it, before the water will run out of the longer leg.

Exp. 6. From what mistake this has proceeded, I cannot imagine; for the water runs out at the longer leg, as soon as the orifice of the shorter leg comes to touch the surface of the stagnant water, without being at all immersed therein.

Jurin's attitude concerning his friend's discovery is pleasing. He appears to have had better judgment than to rush into print, or herald forth that Perpetual Motion had been accomplished. Indeed, the account as given to the Royal Society was that of an experiment and a failure. Nevertheless, it presents an interesting point. Capillary Attraction, however, creates no new energy. Adhesion is a force, and is often quite a strong force in nature.

If a rod or tube be held by the hand at one end, and the other end inserted in a liquid, it will be observed that in some instances, depending upon the nature of the material of the rod or tube, and the liquid, at the point of contact the liquid will slightly rise in the tube and on the outside edges of the tube. In other instances it will be depressed slightly at the same point. Whether it will be elevated or depressed depends on whether the adhesion of the liquid to the material of which the tube or rod is composed is greater than the cohesion of the particles of the liquid.

If there be a depression it is manifest that the entire surface of the liquid will be slightly elevated by reason of the depression. On the contrary, if the liquid adheres to and creeps slightly upward on the tube or rod, then it is manifest that the surface of the liquid will come to rest slightly lower than though it did not so creep.

The net result finally gets back to the principle of flotation. The immersion or insertion is a little more difficult in the case of depression, and a little easier in the case of elevation. There is no gain or loss of energy. It simply increases in one case, and diminishes in the other case the amount of displacement, with all the resulting mechanical phenomena.