Jan. 1 6, 1890] 



AM TURH 



249 



vibration of the earth's crust. My brother Horace and I were, 

 we believe, the first to verify in England the observations of 

 Bertelli, Rossi, d'Abbadie, and the other (principally Italian) 

 pioneers in this interesting subject. 



In our Reports to the British Association for 1881 and 1882 

 on "The Lunar Disturbance of Gravity," some account will be 

 found of the earlier literature on the subject. 



January 9. G. H. Darwin. 



Meteor. 



On Sunday, 12th inst., about 8.10 p.m., a bright meteor 

 was seen here, coming into view near 5 Aurigae. It was of a 

 reddish colour, moved slowly, leaving a short tail, and burst 

 above e Leonis, then with diminished light continued its course 

 to the horizon. T. W. Morton. 



Beaumont College, Old Windsor, January 13. 



MAGNETISMS 

 I. 



A S old as any part of electrical science is the knowledge 

 -^"^ that a needle or bar of steel which has been touched 

 with a loadstone will point to the north. Long before the 

 first experiments of Galvani and Volta the general pro- 

 perties of steel magnets had been observed — how like 

 poles repelled each other, and unlike attracted each other ; 

 how the parts of a broken magnet were each complete 

 magnets with a pair of poles. The general character of 

 the earth's magnetism has long been known — that the 

 earth behaves with regard to magnets as though it had 

 two magnetic poles respectively near the rotative poles, 

 and that these poles have a slow secular motion. For 

 many years the earth's magnetism has been the subject 

 of careful study by the most powerful minds. Gauss 

 organized a staff of voluntary observers, and applied his 

 unsurpassed powers of mathematical analysis to obtaining 

 from their results all that could be learned. 



The magnetism of iron ships is of so much importance 

 in navigation that a good deal of the time of men of 

 great power has been devoted to its study. It was the 

 scientific study of Archibald Smith ; and Airy and 

 Thomson have added not a little to our practical know- 

 ledge of the disturbance of the compass by the iron of 

 the ship. Sir W. Thomson, in addition to much valuable 

 practical work on the compass, and experimental work on 

 magnetism, has given the most complete and elegant 

 mathematical theory of the subject. Of late years the 

 development of the dynamo machine has directed 

 attention to the magnetization of iron from a different 

 point of view, and a very great deal has been done by 

 many workers to ascertain the facts regarding the 

 magnetic properties of iron. The upshot of these many 

 years of study by practical men interested in the mariner's 

 compass or in dynamo machines by theoretical men 

 interested in looking into the nature of things, is, 

 that although we know a great many facts about mag- 

 netism, and a great deal about the relation of these facts 

 to each other, we are as ignorant as ever we were as 

 to any reason why the earth is a magnet, as to why its 

 magnetic poles are in slow motion in relation to its sub- 

 stance, or as to why iron, nickel, and cobalt are magnetic, 

 and nothing else, so far as we know, is to any practical 

 extent. In most branches of science the more facts we 

 know the more fully we recognize a continuity in virtue of 

 which we see the same property running through all the 

 various forms of matter. It is not so in magnetism ; here 

 the more we know the more remarkably exceptional does 

 the property appear, the less chance does there seem to 

 be of resolving it into anything else. It seems to me that 

 I cannot better occupy the present occasion than by re- 

 calling your attention to, and inviting discussion of, some 



" Inaugural Address delivered before the Institution of Electrical En- 

 gineers, on Thursday, January 9, by J. Hopkinson, M.A., D.Sc, F.R.S., 

 President. 



of those salient properties of magnetism as exhibited by 

 iron, nickel, and cobalt — properties most of them very 

 familiar, but properties which any theory of magnetism 

 must reckon with and explain. We shall not touch on 

 the great subject of the earth as a magnet — though much 

 has been recently done, particularly by Riicker and 

 Thorpe — but deal simply with magnetism as a property 

 of these three bodies, and consider its natural history, 

 and how it varies with the varying condition of the 

 material. 



To fix our ideas, let us consider, then, a ring of uniform 

 section of any convenient area and diameter. Let us sup- 

 pose this ring to be wound with copper wire, the convolu- 

 tions being insulated. Over the copper wire let us suppose 

 that a second wire is wound, also insulated, the coils of 

 each wire being arranged as are the coils of any ordinary 

 modern transformer. Let us suppose that the ends of the 

 iimer coil, which we will call the secondary coil, are con- 

 nected to a ballistic galvanometer ; and that the ends of 

 the outer coil, called the primary, are connected, through 

 a key for reversing the current, with a battery. If the 

 current in the primary coil is reversed, the galvanometer 

 needle is observed to receive a sudden or impulsive deflec- 

 tion, indicating that for a short time an electromotive 

 force has been acting on the secondary coil. If the re- 

 sistance of the secondary circuit is varied, the sudden 

 deflection of the galvanometer needle varies inversely as 

 the resistance. With constant resistance of the secondary 

 circuit the deflection varies as the number of convolutions 

 in the secondary circuit. If the ring upon which the 

 coils of copper wire are wound is made of wood or glass 

 — or, indeed, of 99 out of every 100 substances which 

 could be proposed— we should find that for a given 

 current in the primary coil the deflection of the galvano- 

 meter in the secondary circuit is substantially the same. 

 The ring may be of copper, of gold, of wood, or glass-^ 

 it may be solid or it may be hollow — it makes no difference 

 in the deflection of the galvanometer. We find, further, 

 that with the vast majority of substances the deflection of 

 the galvanometer in the secondary circuit is proportional 

 to the current in the primary circuit. If, however, the 

 ring be of soft iron, we find that the conditions are enor- 

 mously different. In the first place, the deflections of the 

 galvanometer are very many times as great as if the ring 

 were made of glass, or copper, or wood. In the second 

 place, the deflections on the galvanometer in the secondary 

 circuit are not proportional to the current in the primary 

 circuit ; but as the current in the primary circuit is step 

 by step increased we find that the galvanometer deflec- 

 tions increase somewhat, as is illustrated in the ac- 

 companying curve (Fig. i), in which the abscissa; are 

 proportional to the primary current, and the ordinates are 

 proportional to the galvanometer deflections. You ob- 

 serve that as the primary current is increased the galvano- 

 meter deflection increases at first at a certain rate ; as 

 the primary current attains a certain value the rate at 

 which the deflection increases therewith is rapidly in- 

 creased, as shown in the upward turn of the curve. This 

 rate of increase is maintained for a time, but only for a 

 time. When the primary current attains a certain value 

 the curve bends downward, indicating that the deflections 

 of the galvanometer are now increasing less rapidly as 

 the primary current is increased ; if the primary current 

 be still continually increased, the galvanometer deflections 

 increase less and less rapidly. 



Now what I want to particularly impress upon you is 

 the enormous difference which exists between soft iron on 

 the one hand, and ordinary substances on the other. On 

 this diagram I have taken the galvanometer deflections 

 to the same scale for iron, and for such substances as 

 glass or wood. You see that the deflections in the case 

 of glass or wood, to the same scale, are so small as to be 

 absolutely inappreciable, whilst the deflection for iron at 

 one point of the curve is something like 2000 times as 



