194 



NATURE 



[December 20, 1900 



A BASE measuring apparatus, which has been perfected in 

 connection with the summer school work of the Civil Engineer- 

 ing Department of the Massachusetts Institute of Technology, 

 has recently been tested by the Coast and Geodetic Survey in 

 Washington. Such satisfactory results have been already 

 obtained that the apparatus is about to be used in the important 

 Lampasas Base in Texas. Prof. Burton, of the Institute, under 

 whose direction the apparatus has been worked out, has been 

 invited to accompany the expedition, which is to make a careful 

 trial of the method in the field and upon extended exact work. 

 The apparatus represents the final results of thesis investigations 

 by several graduates of the course in civil engineering who have 

 worked upon it in successive years. One part of the apparatus 

 maintains a constant tension in the steel tape while in use. 

 Another part of the apparatus determines very accurately the 

 mean temperature of the tape by measuring its electrical resist- 

 ance by means of a special form of thermophone devised by 

 two graduates. 



SOCIETIES AND ACADEMIES. 



London. 



Physical Society, December 14. — Meeting held at the Royal 

 College of Science (by invitation of Prof. Riicker), Principal O. J. 

 Lodge, President, in the chair. — A paper on electric inertia and 

 the inertia of electric convection was read by Prof. A. Schuster. 

 Calculations of self-induction are based on the assumption that 

 the currents which traverse a conductor fill it continuously, the 

 flow being treated as that of an incompressible liquid. The 

 assumption is generally recognised not to hold in the case of 

 electrolytes where electricity is conveyed by a number of irregu- 

 larly distributed ions. In the immediate neighbourhood of such 

 an ion, the magnetic field is many times greater than that calcu- 

 lated on the supposition of continuous distribution, and hence 

 the total magnetic energy is underestimated. What is universally 

 recognised in the case of electrolytes must also be conceded 

 when the current is conveyed by a gas, and the idea is gaining 

 ground that even in solid conductors the current consists of posi- 

 tive and negative electrons moving with different velocities. It 

 is the object of the paper to calculate the additional terms which 

 become necessary for the evaluation of self-induction, and to 

 discuss the possible cases in which the corrections may effect ex- 

 perimental results. The mathematical investigation shows that 

 it is necessary to add a correcting term containing a quantity 

 which may conveniently be called electric inertia. The author 

 has calculated the numerical value 01 this quantity in the case of 

 a solid conductor, and finds it to be about 2x10-1' C.G.S. 

 units. It is of the dimensions ol a surface. The experiments 

 of Hertz have proved that if electric inertia exists, it must be 

 less than iSxio-^ In the case of liquids and gases, the 

 electric inertia of the moving ions becomes much more im- 

 portant, and the calculation of self-induction by the ordinary 

 processes gives erroneous results. The introduction of a term 

 representing inertia alters the general equations of electric 

 motion, and the author has applied his modified theory to 

 Leyden jar discharges, the electrodeless discharges of J. J. 

 Thomson, and the electromagnetic theory of light. In the 

 case of electrodeless discharges in a vacuous globe, it is sug- 

 gested that the absorption of energy may not only be due to the 

 conductivity of the gas, but also to the inertia which it possesses. — 

 A paper on magnetic precession was then read by the same author. 

 The most delicate method of investigating the influence of 

 electric inertia is based on the electromotive forces introduced 

 by the motion of conductors carrying electric currents. If 

 electricity behaves like a body possessing inertia, the rotation 

 of a body through which currents pass should affect the flow 

 of these currents in the same manner as the earth's rotation 

 affects the direction of currents of air. If the earth's magnetism 

 is due to electric currents, it is interesting to see if the effects 

 of inertia can explain the secular variatiorv. The investigation 

 shows that a magnetic precession of the charactej of the secular 

 variation would be produced, but that the precession would 

 be very much slower than the variations actually observed. 

 The subject is worked out mathematically, dealing first with 

 the case of currents in a spherical shell, and then extending the 

 result to the case of a solid sphere. The calculated period 

 qf a cycle comes out as 7 x lo^* years. If the currents are 

 confined to a thin slice of the earth, the time would be reduced 

 to about 14 X 10* years. To produce the actual period of the 



NO. 1625, VOL. 63] 



secular change, the current sheet would have to be of molecular 

 dimensions. This suggests the possibility of the phenomenon 

 of secular variation being rather of a molecular than a molar 

 character. Prof. Riicker congratulated the author upon his at- 

 tempt to solve the problem of terrestrial magnetism, and ex- 

 pressed the hope that further calculation would throw more 

 light upon this difficult subject. Mr. Blakesley asked if the 

 time of the secular variation would be altered if the interior of 

 the earth were liquid or solid. The chairman observed that 

 the precession was rapid in the case of a thin layer of gas, and 

 mentioned J. J. Thomson's notion that the electrons were thrown 

 off by centrifugal force and formed a molecular layer. Hertz, 

 in his experiments on electricity, had looked for material inertia 

 besides electromagnetic inertia. In the present theory the dis- 

 tinction disappears, and there is only one inertia, and that electro- 

 magnetic. Prof. Ayrton said if the two forms of inertia were 

 electromagnetic, he would like to know why, in detecting 

 the second form, it was necessary to associate it with an absorp- 

 tion of energy, as in the case of an electrodeless discharge. In 

 the case of ordinary self-induction there is no absorption of 

 energy, and if there is absorption in the second form, and if 

 they are both electromagnetic, he would like to know the dif- 

 ference between the two. Prof. Schuster, replying to Mr. 

 Blakesley, said that if the interior of the earth were treated as 

 liquid, the period of the cycle would be about one hundred times 

 less. In reply to Prof. Ayrton, he said he had only cited one 

 experiment to show that a phenomenon, explained by the gas 

 being a good conductor, could also be explained by its electric 

 inertia. It was impossible to say in general whether self- 

 induction caused an absorption of energy or not.— Prof. A. W. 

 Riicker read a paper on the magnetic field produced by electric 

 tramways. Taking the case of a tramway in which the current 

 flows along a trolley wire from the power-house, and returns 

 partly through the rails and partly as earth currents, the author 

 has shown that the vertical disturbing force at any point is due 

 to the currents in the feeders and rails, and that the earth cur- 

 rents affect the horizontal force only. Experiment shows that it 

 is chiefly the vertical force instruments which are affected by 

 the establishment of an electric railway, and since this disturb- 

 ance is due to the wires and rails it is impossible for an 

 observatory to be protected by rivers or other natural features of 

 the district. A preliminary investigation is based on the assump- 

 tion that the trolley wires and rails are insulated conductors, and 

 that a fraction of the whole current returns along the rails to the 

 generator. The effect of the railway at a distant point 

 is due to the difference of the current in the trolley 

 wire and the hypothetical uniform rail current, the effect 

 of which at the point considered is equivalent to 

 the actual rail current, which varies from point to point. 

 It is thus shown that the disturbance increases with the length 

 of the tramway, and for a tramway of given length the disturb- 

 ance is a maximum at points on a line perpendicular to and 

 bisecting it. Experiments made at Stockton on the magnitude 

 of the disturbing force gave, with the vertical force instrument, 

 a leakage of 16 '3 per cent., and with the horizontal force 

 instrument a leakage of 15 '9 per cent., a fairly close agree- 

 ment. The assumption that the terminals of the line are 

 above and below the average potential of the earth by the same 

 amount respectively, and that the leakage at any point is 

 proportional to the potential difference between the rail and the 

 earth, leads to the ordinary theory of a Fourier bar. This more 

 accurate assumption has been developed and applied to the 

 results obtained at Stockton. The leakage, as calculated from 

 the amperes and volts, comes out as 20 per cent. The calculation 

 of the disturbing vertical force gives iO"5 x 10"^ C.G S. units, 

 which is in fair agreement with the value 7 x io~^ actually 

 observed. In conclusion, it is pointed out that for practical 

 purposes it is sufficient to deal with the average return current 

 through the rails, the formulae for which are quite simple. — Dr. 

 R. T. Glazebrook read some notes on the practical applica- 

 tion qf the theory of magnetic disturbances by earth currents. 

 In this paper the author has thrown the extended formula 

 obtained by Prof. Riicker in the previous paper into a workable 

 form, and has tabulated numbers which show at what distances 

 the disturbances are negligible for tramways of different lengths. 

 — Prof. R. Threlfall exhibited a quartz-thread gravity balance. 

 Prof. Threlfall gave a short description of this instrument, 

 which has been described in full elsewhere. He drew 

 attention particularly to its accuracy and portability. 

 Mr, , Simpson asked how far the fibre had been calibrated, and 



