frequently met with cases where the presence of a conductor diminishes the intensity of the discharge. One of the most striking of these is when the two jars are insulated, and a square discharge tube used. The spark was adjusted so that the discharge just, but only just, went round the tube. A sphere connected to earth was then moved round the discharge tube; in some positions it increased the brilliancy of the discharge, and the tube became quite bright, while in other positions it stopped the discharge altogether. The observation of the behavior of the discharges through these tubes is a very convenient method of studying the effect of conductors in deflecting the flow of the tubes of electrostatic induction which fall upon them; for the appearance of the discharge is affected not merely by the average, but also by the maximum value of the electro-motive intensity which produces it. Thus a high maximum value, lasting only for a short time, might produce a discharge, while a more equable distribution of electro-motive intensity having the same average value might leave the tube quite dark. FIG. 15. B I have employed these discharges to study the behavior of bodies under the action of very rapid electrical oscillations in the following way: In the primary circuit connecting the outside coatings of the jar two loops, A and B (Fig. 15), were made, in one of which, A, an exhausted bulb was placed, the sparklength and the pressure of the gas on it being adjusted until the discharge was sensitive, i. e., until a small alteration in the electro-motive intensity acting on the bulb produced a considerable effect upon the appearance presented by the discharge. The substance whose behavior under rapid electrical vibrations was to be examined was placed in the loop B. The results got at first were very perplexing, and at first sight contradictory, and it was some time before I could see their explanation. The following are some of these results: When a highly exhausted bulb was placed in B a brilliant discharge passed through it, while the discharge in A stopped. A bulb of the same size, filled with a dilute solution of electrolyte, produced no appreciable effect; when filled with a strong solution it dimmed the discharge in A, but not to the same extent as the exhausted bulb. A piece of brass rod or tube increased the brightness of the discharge in A; on the other hand, a similar piece of iron rod or tube stopped the discharge in A at once. The most decided effect, however, was produced by a small crucible made of plumbago and clay; this, when put in the loop B, stopped the discharge in A completely. I found however that by considering the work spent on the substance placed in B, the preceding results could be explained. When a large amount of work is spent in B, the discharge in A will be dimmed, while no appreciable effect will be produced on A when the work spent in B is small. Now let us consider the work done in a secondary circuit whose resistance is R, whose coefficient of self-induction is L, and which has a coefficient of mutual induction, M, with the primary circuit. If the frequency of the current circulating in the primary is p, we can easily prove that the rate of absorption of work by the secondary is proportional to R M2 p2 Thus the work given to the secondary vanishes when R=0 and whet Rinfinity, and has a maximum value when R=Lp. Thus the condition that the secondary should absorb a considerable amount of work is that the resistance should not differ much from a value depending on the shape of the circuit and the frequency of the current in the primary. No appreciable amount of work is consumed when the resist ance is very much greater or very much less than this value. I tested this result by placing inside B a coil of copper wire. When the ends were free, so that no current could pass through it, it produced effect upon the bulb in A; when the ends were joined so that there was only a very small resistance in the circuit, the effect was, if any thing, to increase the brightness of the discharge in A. When however the ends were connected through a carbon resistance which could be adjusted at will, the discharge in A became very distinctly duller when there was a very considerable resistance in the circuit. This experiment confirms the conclusion that to absorb energy the resist ance must lie within certain limits, and be neither too large nor to small. We can now see the cause of the differences observed when the substances mentioned above were put into B. The brass rod and tube did not dim the discharge in A, because their resistance was too low; the weak solution of electrolyte, because the resistance was too great; while the resistances of the crucible and the strong solution of elec trolyte which obliterated the discharge from A were near the value for which the absorption of energy by the system was a maximum. The case of iron is very interesting because it shows that even under these very rapidly alternating forces, iron still retains its magnetic properties. A striking illustration of the difference between iron and other metals is shown when we take an iron rod and place it in B, the discharge in A immediately stops; if we now slip a brass tube over the iron rod the discharge in A is at once restored. If on the other hand we use a brass rod and an iron tube, when the rod is put in B without the tube the discharge in A is bright; if we slip the iron tube over the rod, the discharge stops. To compare the amount of heat produced in the brass and iron secondaries [calculations are introduced by which the author estimates that] for iron and copper cylinders of the same dimensions it would be about seventy times as large in the iron as in the copper, assuming that the iron retains its magnetic properties under these very rapidly alternating forces. The result explains the effect of the iron in stopping the discharge. As I am not aware that any magnetic properties of iron under such rapidly alternating forces have been observed, I was anxious to make quite sure that the difference between iron and brass was not due solely to the differences between their specific resistances. The first experiment I tried with this object was to cover the iron rod with thin sheet platinum, such as is used for Grove cells. As the resistance of platinum is not very different from that of iron, if the effect depended merely upon the resistance, slipping a thin tube of platinum over the iron ought to make very little difference. I found however that when the platinum was placed over the iron, all the peculiar effects produced by the latter were absent, thus showing that the effect is not due to the resistance of the iron. It then occurred to me that I might test the same thing in another way by magnetizing the iron to saturation, for in this state u is nearly unity; thus if the result depended mainly on the magnetic properties of the iron it ought to diminish when the latter is strongly magnetized. I accordingly tried an experiment in which the iron in the coil B was placed between the poles of a powerful electro-magnet. When the magnet was "off" the iron almost stopped the discharge in A; when it was "on" the discharge became brighter, not indeed so bright as if the iron were away altogether, but still unmistakably brighter than when it was unmagnetized. This experiment, I think, proves that iron retains its magnetic properties when exposed to these rapidly alternating forces. Another result worthy of remark is that though a brass rod or tube inserted in B does not stop the discharge in A, yet if a piece of glass tubing of the same dimensions is coated with Dutch metal, or if it has a thin film of silver deposited upon it, it will stop the discharge very decidedly. We are thus led to the somewhat unexpected result that a thin layer of metal when exposed to these very rapid electrical vibrations may absorb more heat than a thick one. I find, on calculating the heating effect in slabs of various thicknesses, that there is a thickness for which the heat absorbed is a maximum. The slight increase in the brightness of the discharge in A when a brass rod is placed in B is due, I think, to the diminution in the selfinduction in the primary circuit produced by this rod whose conductivity is so good that it absorbs practically no heat. We will now return to the case of bad conductors, where na is small; here the absorption of energy is proportional to the conductivity, and we might use this method to compare the conductivity of electrolytes for very rapidly alternating currents. I tried a few experiments of this kind and found, as I did in the experiments described in the Proceedings of the Royal Society, XLV, p. 269, that the ratio of the conductivities of two electrolytes was the same for rapidly alternating as for steady currents. I was anxious, however, to see whether these rapidly alternating currents could pass with the same facility as steady cur rents from an electrolite to a metal. To try this two equal beakers were filled with the same electrolyte made of such strength that when inserted in B they put out the discharge in A. I then placed in one beaker six ebonite diaphragms arranged so as to stop the eddy cur rents, and a similar metallic diaphragm in the other. The ebonite diaphragm made the beaker in which it was placed cease to have any effect upon the discharge in A. I could not detect however that the effect of the beaker in which the metal diaphragm was placed on the discharge in A was at all diminished by the introduction of the dia phragm. I conclude therefore that very rapidly alternating currents can pass with facility from electrolytes to metals and vice versa. In this respect electrolytes differ from gases, the currents in which, as we have seen, are stopped by a metallic diaphragm in the same way as they would be by an ebonite one. It may be useful to observe in passing that a somewhat minute division of the electrolyte by the non-conducting diaphragm is necessary to stop the effect of the eddy currents; a division of the electrolytes into two or three portions seemed to produce very little effect. Another point which is brought out by these experiments is the great conductivity of rarified gases when no electrodes are used as compared with that of electrolytes. An exhausted bulb will produce as much effect on the discharge in A as the same bulb filled with a solution of an electrolyte containing about a hundred thousand times as many molecules of electrolyte. The molecular conductivity of rarified gases when the electro-motive intensity is very great and when no electrode are used must be thus enormously greater than that of electrolytes. Bulbs filled with rarified gas used in the way I have described serve as galvanometers, by which we can estimate roughly the relative in tensity of the current flowing through the primary coils which encircle them. Used for this purpose I have found them very useful in some experiments on which I am at present engaged, on the distribution of very rapidly alternating currents among a net-work of conductors. THE MOLECULAR PROCESS IN MAGNETIC INDUCTION.* By Prof. J. A. EWING, F. R. S. Magnetic induction is the name given by Faraday to the act of becoming magnetized, which certain substances perform when they are placed in a magnetic field. A magnetic field is the region near a magnet, or near a conductor conveying an electric current. Throughout such a region there is what is called magnetie force, and when certain substances are placed in the magnetic field the magnetic force causes them to become magnetized by magnetic induction. An effective way of producing a magnetic field is to wind a conducting wire into a coil, and pass a current through the wire. Within the coil we have a region of comparatively strong magnetic force, and when a piece of iron is placed there it may be strongly magnetized. Not all substances possess this property. Put a piece of wood or stone or copper or silver into the field, and nothing noteworthy happens; but put a piece of iron or nickel or cobalt and at once you find that the piece has become a maghet. These three metals, with some of their alloys and compounds, stand out from all other substances in this respect. Not only are they capable of magnetic induction-of becoming magnets while exposed to the action of the magnetic field, but when withdrawn from the field they are found to retain a part of the magnetism they acquired. They all show this property of retentiveness, more or less. In some of them this residual magnetism is feebly held, and may be shaken out or otherwise removed without difficulty. In others, notably in some steels, it is very persistent, and the fact is taken advantage of in the manufacture of permanent magnets, which are simply bars of steel, of proper quality, which have been subjected to the action of a strong magnetic field. Of all substances, soft iron is the most susceptible to the action of the field. It can also under favorable conditions, retainwhen taken out of the field-a very large fraction of the magnetism that has been induced-more than nine-tenths,-more indeed than is retained by steel; but its hold of this residual magnetism is not firm, and for that reason it will not serve as a material for permanent magnets. My purpose to-night is to give some account of the molecular process through which we may conceive magnetic induction to take place, and of the structure which makes residual magnetism possible. Abstract of a Friday evening discourse delivered at the Royal Institution on May 22, 1891. (From Nature, Oct. 15, 1891; vol, XLIV, pp. 566–572.) |