Kelvin's syphon recorders, made by Messrs. Muirhead & Co., which will be at work after the lecture, and also an artificial cable, which has also been made by Messrs. Muirhead & Co. Curves can be obtained from the end and middle of the cable. Magnetic experiments such as these have really a wide application. They are not restricted to the particular size of the cylinder upon which the experiment is made. If the cylinder is larger or smaller similar magnetic events will happen, but they will happen at times shorter or longer in direct proportion to the square of the linear dimensions of the cylinders-that is, to the areas of the cylinders. Hence we may boldly infer what will happen in magnet cores of dimensions too small to experiment upon or so large that they would be very costly to make. For example, suppose a core is FIG. 12. made up of wires of an inch in diameter, we may expect that everything will go on with the same currents around the core but 1,440,000 times as fast; with a force of 24, everything will be over at the centre of the core if the current is instantaneously reversed in 40000 of a second. One very practical application of these results is to the cores of transformers, but in this case we have not the sudden reversal of the magnetising current, but the current continuously varies from positive to negative and from negative to positive. It is not difficult to imitate these conditions on the slow scale which is suitable for our large magnet, but if I were to show you an experiment at an appropriate rate on the magnet which we used for reversals, I fear you would find the results intolerably tedious. It would take some minutes to get through a single reversal. I will, therefore, use VOL. XIV. (No. 89.) 2 P a smaller magnet, 4 inches instead of 12 inches diameter, which will get through the business nine times as speedily. I w! first of all throw upon the screen a diagram (Fig. 13) which shows the method of the experiment. Here is the source of electricity. marked “dynamo"; here are resistances for regulating the current, marked "rheostat"; and here is a liquid reverser intended to reverse the direction of the current by continuous steps. It er sista of two copper plates, to which the current is taken, and of twe moving plates revolving between these plates, which plates are connected to the electromagnet. As the moving plates revolve. 2 current will diminish from a maximum in one direction to niz and will then increase to a maximum in the opposite direction. : electromagnet is marked "magnet," the secondary coils of which ar not indicated in the diagram. Only one coil in the centre of magnet is connected. We will now turn the reverser faster. I+ see, the variations of inductions diminish greatly indeed, the duction at the centre of the core is but little affected by the chang current in the magnetising coils (change leads). We might b tried three coils instead of one, but the experiment is a confusing. If we had done so, we should have seen that the diminution of disturbance with increase of speed was less with No. 2 coil than with No. 1 coil, and that it had disappeared entirely with No. 3 coil, and we should further see that the current in No. 2 coil lay behind No. 3, and in No. 1 behind No. 2. I can show you the contrast more effectively with the Westinghouse transformer with a divided core. This is now connected to the lowest galvanometer. We turn slowly notice the deflection. We turn faster: you see the deflection is increased instead of diminished, as it was with the central coil of the solid electromagnet. Experiments such as these are at once applicable to transformer cores and the cores of dynamo machines. They show that in practice manufacturers have divided the iron about enough, and not too much. να FIG. 14.-MAGNETIC CURVE-TRACER CURVES FOR SOFT IRON BARS. (a) Cycle performed slowly. (c) Period of cycle 0·43 second. I have here on the table an instrument designed by Prof. Ewing for the purpose of describing the curves which express the relation between induction and magnetising force in iron. It served in Prof. Ewing's hands to illustrate the point which we have been discussing. I will throw upon the screen curves taken from Prof. Ewing's paper read before the Royal Society. The curves have been taken from solid samples of iron-I mean iron which is con tinuous, and not divided for the purpose of annulling the current in it. In the first curve (a, Fig. 14) we have the result in which the cycle has been passed through very slowly, and is the true curve of magnetisation. We have b, where the cycle has passed through in three seconds-and you will observe that the amplitude of the 552 On the Effects of Electric Currents in Iron, &c. [April 26, induction has diminished, and we have, lastly, the curve c, where the period of the cycle is four-tenths of a second, and you will observe that it has much more diminished. This is owing to the fact that changes of induction in the centre of the iron hardly exist, and that therefore the total effect is materially diminished. The diagram (Fig. 15) is for steel. We have a the natural curve of induction; b the case when the period of the cycle is three seconds; and c when the period is six-tenths of a second. Curves such as these, of course, very readily give you the average effects upon the whole mass of the iron. The curves which I have shown you give the particular effects of different parts of the mass. a FIG. 15.-MAGNETIC CURVE-TRACER CURVES OF STEEL. (a) Cycle performed slowly. In conclusion, let us indulge in a little wild speculation, not because it is probable that it is in any sense true, but because it is interesting. Suppose a magnet were made exactly like the one on which we experimented, but of the size of the earth, and that some mighty electrician generated such a current in its copper coils as would give a magnetising force of 2.5, and then reversed it, it would take some thousands of millions of years before the rate of disturbance at the centre attained its maximum value. The speculation I suggest is this: is it not conceivable that the magnetism of the earth may be due to currents in its material sustained by its changing induction but slowly dying away? [J. H.] ANNUAL MEETING, Wednesday, May 1, 1895. SIR JAMES CRICHTON-BROWNE, M.D. LL.D. F.R.S. Treasurer and The Annual Report of the Committee of Visitors for the year 1894, testifying to the continued prosperity and efficient management of the Institution, was read and adopted. The Real and Funded Property now amounts to above 102.000l. entirely derived from the Contributions and Donations of the Members and of others appreciating the value of the work of the Institution. Sixty-two new Members were elected in 1894. Sixty-three Lectures and Nineteen Evening Discourses were delivered in 1894. The Books and Pamphlets presented in 1894 amounted to about Thanks were voted to the President, Treasurer, and the Honorary The following Gentlemen were unanimously elected as Officers for the ensuing year: PRESIDENT―The Duke of Northumberland, K.G. D.C.L. LL.D. MANAGERS. Sir Frederick Abel, Bart. K.C.B. D.C.L. LL.D. Captain W. de W. Abney, C.B. D.C.L. F.R.S. Edward Frankland, Esq. D.C.L. LL D. F.R.S. George Matthey, Esq. F.R.S. VISITORS. John Wolfe Barry, Esq. C.B. M. Inst. C.E. Carl Haag, Esq. R.W.S. Victor Horsley, Esq. M.B. F.R.S. F.R.C.S. Sir Joseph Lister, Bart. M.D. D.C.L. LL.D. Lachlan Mackintosh Rate, Esq. M.A. Henry Virtue Tebbs, Esq. Silvanus P. Thompson, Esq. D.Sc. F.R.S. The Right Hon. The Marquis of Salisbury, K.G. John Westlake, Esq. Q.C. LL.D. D.C.L. LL.D. F.R.S. Basil Woodd Smith, Esq. F.R.A.S. F.S.A. Joseph Wilson Swan, Esq. M.A. F.R.S. His Honour Judge Frederick Meadows White, Q.C. Sir William H. White, K.C.B. LL.D. F.R.S. |