Abstract
Physical Society, October 27.—Prof. W. E. Ayrton, F.R.S., Vice-President, in the chair.—Dr. S. W. Richardson read a paper on the magnetic properties of the alloys of iron and aluminium. Observations were made upon four alloys containing respectively 3.64, 5.44, 9.89 and 18.47 per cent. of aluminium. The alloys were used in the form of anchor rings, and were wound with primary and secondary coils separated by asbestos paper. The temperatures used ranged from –83° C. to 900° C. The low temperatures were produced by the rapid evaporation of ether surrounded either by ice and salt or by carbon dioxide snow. The high temperatures were obtained either electrically or by gas muffles. In both cases the actual temperatures were deduced from the resistance of the secondary, which was made of platinum wire and wound next the metal. The author employed Maxwell's null method of measuring mutual induction, increasing the sensitiveness by the introduction of a secohmmeter making about three revolutions per second. In order to test the accuracy of the method some of the experiments were repeated with a ballistic galvanometer in the ordinary way, and the agreement obtained between the results in the two cases was well within the limits of experimental error. The chief conclusions to be drawn from the experiments may be summed up as follows: (1) The alloys behave magnetically as though they consisted of two distinct media superposed. (2) The general roundness of the curves and their lack of abruptness near the critical point seems to indicate that the alloys are heterogeneous in structure. (3) The permeability decreases with rise of temperature near the critical point until a minimum value is reached, when further rise of temperature produces very slight diminution, if any, in the permeability. (4) The experiments suggest that the maximum value of the permeability for an alloy containing 10 per cent, of aluminium is reached at about –90° C. (5) An alloy containing 18.47 per cent, of aluminium has a critical point at about 25° C., and gives no indication of temperature hysteresis. This alloy probably has a maximum permeability much below –90° C. The author has found that at high temperatures there is a second maximum on the induction curve. This maximum becomes less and less noticeable as the field is increased.—The Secretary read a note from Prof. Barrett on the electric and magnetic properties of aluminium and other steels. The first part of the note dealt with the electrical conductivity of various alloys, and discussed the effect of composition and annealing upon the value of the conductivity. The second part of the note referred to magnetic effects. The most remarkable effect produced by aluminium on iron is the reduction of the hysteresis loss. The permeability of nickel steels is shown to be very much influenced by annealing. It is found that the addition of a small quantity of tungsten to iron hardly affects the maximum induction, yet increases the retentivity and coercive force. The experiments show that the best steel for making permanent magnets is one containing 7½ per cent, of tungsten. The magnetometric method was employed throughout. Prof. S. P. Thompson drew attention to the wide range of temperature over which the author had conducted his experiments, and also to the small number of alloys used. He said a very much finer connection between the properties could be obtained from the examination of more alloys, and expressed his interest in the existence of the second maximum on the induction curve. He would like to know how the percentage composition of the alloys had been determined. Turning to Prof. Barrett's note, Prof. Thompson referred to the difference in the breadths of the hysteresis curves for aluminium and chromium alloys. Mr. Appleyard asked for information upon the permanence of the curves. Dr. Richardson, in replying, said the compositions were determined by analyses made after the experiments had been performed. It was proposed to carry on the research upon a series of aluminium alloys which he had obtained. The Chairman expressed his special interest in the agreement which the author had obtained between the ballistic method and the null method of Maxwell increased in sensitiveness by the secohmmeter.—Mr. Addenbrooke exhibited a model illustrating a number of the actions in the flow of an electric current. The model consisted of a spiral of steel wire in the form of a closed circuit. Inside the spiral was placed a wire which was supposed to be carrying the current, and which directed the motion of the spiral. A rotational movement given to one part of the spiral was transmitted by the wire, and produced a rotational movement at another part of the spiral. The resiliency of the spring represents capacity, and the torque electromotive force. Self-induction can be represented by weighting the spring. Prof. Everett expressed his interest in the way that the correspondence between the propagation and rotation agreed with that between the direction of a current and the direction of the magnetic force. Prof. S. P. Thompson agreed that many analogies could be worked out by the model, but gave one. or two examples to show that erroneous conclusions might be drawn by pushing the analogy too far.—Mr. W. Watson repeated some experiments with the Welinelt interrupter devised by Prof. Lecher. The experiments showed in a clear and striking manner the fact that subsequent sparks tend to pass through the portion of air heated by the first one. In the first experiments motion of the heated air was caused by differences in density, and in the later experiments by allowing the sparks to take place in a strong electromagnetic field. The continuous rotation of the spark in a given field proved the unidirectional nature of the discharge. In reply to Mr. Blakesley, Mr. Watson said he used the word “ionised” in his explanations to express simply the fact that the air had been rendered a conductor by the passage of the spark. The Chairman referred to one of the first experiments performed. In this experiment the electrodes consisted of two copper wires in a vertical plane, slightly inclined to one another and nearest together at their lowest points. On switching on the current the spark passed between the lowest points; but as the ionised air ascended so did the most conducting path, and consequently the spark worked its way to the top of the electrodes. Here the heated air passed away and the spirk returned to the lowest point to rise again. The Chairman thought that these effects might be due to the magnetic forces produced by the circuit itself. That similar effects in the are light were due to this cause had been proved many years ago. Mr. Watson repeated some of the experiments under new conditions, and proved that the explanation of the phenomena was not to be found in the tendency of the circuit to enlarge itself owing to magnetic forces. Mr. Boys pointed out that the relation of the heating effect to the current, which was small in the are light, was very large in the case of the spark discharges used, and therefore the movement of the spark in the latter case was practically determined by the heating effect in consequence of the relatively small importance of the electromagnetic effect. Prof. S. P. Thompson remarked that similar effects could be produced by an alternating current working an ordinary induction coil.—The Society then adjourned until November 10, when the meeting will be held in the Central Technical Institute.
Article PDF
Rights and permissions
About this article
Cite this article
Societies and Academies . Nature 61, 23–24 (1899). https://doi.org/10.1038/061023a0
Issue Date:
DOI: https://doi.org/10.1038/061023a0