64 



NA TURE 



[June i6, 1892 



obtained in some investigations on the time-rate of rise of 

 cunent in a circuit having large electromagnetic inertia. The 

 experiments were made on a circuit containing the coils of a 

 large electromagnet having laminated cores and pole pieces. 

 The mean length of the iron circuit was about 250 cm,, and its 

 cross section 320 sq. cm. I'he magnetizing coil had 3840 

 turns, when all joined in series, and a resistance of io"4 ohms. 

 The coils were so arranged that they could be joined in a 

 variety of ways so as to vary the resistance, inductive co- 

 efficient, &c., and also to allow the magnet to be used either as 

 an open or a closed circuit transformer. 



The electromotive force used in the experiments was ob- 

 tained from a storage battery, and the method of experiment 

 was to trace the curve, giving the relation of current to time, on 

 a cronograph sheet. 



One set of experiments shows the effect of varying the im- 

 pressed E.M.F. on the time required for the current to attain 

 any given percentage of its maximum strength. The results 

 show that for any particular percentage there is always a 

 particular E.M.F. which takes maximum time. Thus for the 

 circuit under consideration, and with successive repetitions of 

 the current in the same direction, it takes longer time for the 

 current produced by an impressed E.M.F. of 4 volts to reach 

 95 per cent, of its maximum than it takes for the current pro- 

 duced by either 3 or 5 volts to reach 95 per cent, of their 

 maximum. The results show also that, within considerable 

 limits, the time required for the current to become uniform is on 

 the whole nearly inversely proportional to the impressed 

 E.M.F., and that for moderate values of the E.M.F. the 

 time may be very great; when the E.M.F. was 2 volts, and 

 the current sen: in such a direction as to reverse the mag- 

 netism left in the magnet by a previous current of the sam'e 

 strength, the time required for the current to establish itself 

 was over three minutes. The difference of time required for 

 repetition and for reversal of previous magnetization was also 

 very marked when the iron circuit was closed. The results 

 show that great errors may arise by the use of ballistic methods 

 of experiment, especially when weak currents are used, and that 

 for testing resistances of circuits containing electromagnets, a 

 saving of time may be obtained by using a battery of consider- 

 able E.M.F. 



Another set of experiments gives the effect of successive 

 reversals of the impressed E.M.F. at sufficient intervals apart 

 to allow the magnetization to be established in each direction 

 before reversal began. In this set also the effect of cutting out 

 the battery and leaving the magnet circuit closed is illustrated, 

 showing that several minutes may be required for the magnet to 

 lose its magnetism by dissipation of energy in the magnetizing 

 coil. The effect on these cycles of leaving an air space in the 

 iron circuit is also illustrated. It is shown that a comparatively 

 small air space nearly eliminates the residual magnetism, and 

 diminishes considerably the rate of variation of the coefficient of 

 induction and the dissipation of energy in the magnet. 



Several cycles are shown for the magnet used as a transformer 

 with different loads on the secondary. The results give evi- 

 dence that there is less energy dissipated in the iron the greater 

 the load on the secondary of the transformer. 



Some experiments are also quoted which go to show that the 

 dissipation of energy due to magnetic retentiveness (magnetic 

 hysteresis) is simply proportional to the total induction produced 

 when the measurements are made by kinetic methods. Refer- 

 ence is made to the recent experiments of Alexander Siemens 

 and others which seem to confirm this view. 



Physical Society, May 27. — Mr. Walter Baily, Vice- 

 President, in the chair. — The following communication was 

 read : — On the present state of our knowledge of the con- 

 nection between ether and matter : an historical summary, by 

 Prof. O.J. Lodge, F.R.S. Referring to difficulties connected 

 with the aberration of light, if the medium were supposed to be 

 cariied along by the earth in its orbit, Dr. Lodge described 

 Boscovich's suggested experiment with a telescope filled with 

 water, carried out by Klinkerfues, who was led to conclude that 

 the aberration constant depended on the medium within the 

 telescope. Klinkerfues's experiments were repeated by Sir G. B. 

 Airy, but not confirmed. Astronomical observations were not 

 necessary to determine the point at issue, for a fixed source near 

 a collimator might be used with advantage. Hoek had examined 

 the subject in this way with similar negative results. It might 

 therefore be concluded that surveying operations are unaffected 

 by terrestrial motion. This result, however, did not prove the 



NO. II 8 1 , VOL. 46] 



existence or non-existence of an ether drift relative to the earth, 

 for, since the source and receiver move together, any effect pro- 

 duced by such a drift would be compensaitd by aberration due 

 to motion of the receiver. Speaking of refraction, he pointed 

 out that, if the ether were stationary in space, glass and other 

 terrestrial bodies would have ether streaming through them, and 

 that the refraction of, say, glass might difler as the direction of 

 the ether drift through it varied. To test this, Arago placed an 

 achromatic prism over the object-glass of a telescope on a mural 

 circle, and observed the altitude of stars. To vary the direction 

 of the ether drift through the prisms, stars in different azimuths 

 were observed ; hut the results showed no appreciable change in 

 the deviation produced by the prism due to direction of the 

 earth's motion. Maxwell used a spectroscope to test the same 

 point. Light from illuminated cross wires passed through the 

 telescope, prisms and collimator, and was reflected back along 

 the same path by a mirror and viewed through the telescope. 

 Observations made with different aspects of instrument showed 

 no change in the relative positions of the wires and their images. 

 Mascart had also tried the experiment with simpler apparatus, 

 but was unable to detect any change. The-e observations 

 naturally suggest that the ether is at rest relative to the earth, 

 but the apparently simple nature of aberration makes this view 

 difficult to hold. Both phenomena are consistent with Fresnel's 

 hypothesis that only the excess of ether, which the substance 

 possesses over that of surrounding space, moves with the body, 

 for on this supposition the effects of altered refraction and ether 

 drift compensate each other. Fresnel's view is practically 

 established by Fizeau's well-known experiment on the effect of 

 moving water on the velocity of light, and by the more accurate 

 numerical results obtained by Michelson. The only other theory 

 which accounts for the experimental results is one by Prof. J. J. 

 Thomson, which requires that the velocity of light in Fizeau's 

 experiment should be altered by half the velocity of the medium. 

 For media whose refractive indices are -^2, the two theories lead 

 to the same result, and as the indices of substances such as water 

 do not differ much from this value, it is difficult to discriminate 

 between them. Looked at in another way, Fizeau's experiment 

 raises a difficulty, for, as Dr. Lodge pointed out, all water is 

 moving with the earth, hence light should be hurried or hindered 

 according to the direction in which it passes through the water. 

 This effect doubtless exists, but the results of it have never been 

 detected by experiment. It is therefore necessary to inquire 

 why the effect could not be observed directly, for the experiment 

 had been tried with interference apparatus by Babinet, Hoek, 

 Jamin, and Mascart, and in no case was any effect observed. 

 It would therefore seem as if the ether must be stagnant, i.e. 

 stationary relative to the earth. Mascart had also tested whether 

 Newton's rings, or the rotary power of quartz, were affected by 

 ether drift, but with negative results. These observations are, 

 however, likewise compatible with Fresnel's hypothesis of an 

 ether fixed relative to matter, and a free ether of space permeat- 

 ing all substances, for, according to this view, there is no more 

 motion of the ether in water or glass than in air ; hence the 

 time of journey round a closed contour is independent of the 

 direction in which the light traverses that contour. The time of 

 journey between two points is also unaffected by terrestrial 

 motion, as was proved by the experiments of Babinet, Hoek, 

 and Mascart on interference ; hence he (Dr. Lodge) inferred 

 that ether was either stagnant or had a velocity potential. In 

 moving ether it was necessary to define a ray, and Lorentz's 

 method is the best. Suppose CP represents the velocity of 



light (V) in still ether, and SC the velocity of the ether {v), then 

 a disturbance originating at S will travel along SP, which is the 



