transactions of the sections. 38 



Electricity. 

 On the Viscosity of Dielectrics, By W, E, Aybton and J. Pebby. 



On the Contact Theory of Voltaic Action. By W. E. Ayeton and J. Peeey. 



On Magnetic Induction as affecting Observations of the Intensity of the 

 Horizontal Component of the Earth's Magnetic Force. By Chaeles 

 Chambebs, F.B.S., Superintendent of the Colaba Observatory, Bombay. 



All our experiments for the measurement of the earth's magnetic force are neces- 

 sarily made in the field of that force, consequently the magnets used in observations 

 of deflection and vibration are subject to the inducing action of that force ; and 

 it is the universal practice of magnetic observatories, sanctioned by the most 

 eminent writers on terrestrial magnetism, to apply corrections on accoimt of induc- 

 tion both to the deflection and vibration observations. The object of this com- 

 munication is to advance theoretical reasons — supported by experimental evidence — 

 against the propriety of the particular correction applied to the vibration observa- 

 tion. This correction is based on the assumption that the vibration-magnet is 

 susceptible of induction longitudinally but not transversely, or not so sensibly ; and 

 the assumption probably rests on what the writer regards as a false analogy between 

 a permanent magnet and an induced magnet. The former, when removed from 

 the influence of a strong magnetizing action, remains a magnet by virtue of its own 

 internal forces, whilst the latter is a magnet by virtue of external forces alone ; 

 it does not therefore follow that because the power of a permanent magnet — mea- 

 sured by its magnetic moment — cannot be made by the same means nearly as great 

 transversely as longitudinally, therefore the same may be said of an induced magnet. 

 Indeed, in his treatment of the subject of the deviations of the compass, Sir 

 George Airy gives to each elemental portion of a ship's iron as great a suscepti- 

 bility to induction in one direction as in another ; and in the more elaborate treat- 

 ment of the same subject, in which Poisson's equations are taken as expressing the 

 fundamental conception of the theory, terms representing transverse induction are 

 still retained, as of comparable magnitude, in presence of others representing longi- 

 tudinal induction. 



Applying the Astronomer Eoyal's theory to the particular case of the vibration 

 magnet, its induced magnetism becomes an assemblage of elementary magnets 

 whose magnetic axes are all parallel to the magnetic meridian, and which, since 

 they sensibly retain their parallelism to the meridian during the oscillation of the 

 magnet, give rise to no moment of restitution ; hence, according to this view, no 

 correction would be required. According to Poisson's theory the amount of the 

 correction is matter for experimental inquiry, and cannot be safely determined on 

 a priori ground. It may be objected, however, that the swinging of a ship being 

 a slow motion compared with the oscillation of a magnet, the theory of the devia- 

 tions of the compass must be modified in its application to the case in question ; 

 and this is no doubt a correct view ; for the theory regards the inductive action as 

 being, at every moment considered, sensibly carried to its limit of effectiveness ; 

 whilst it is not only conceivable, but doubtless the fact, that where, as with the 

 oscillating magnet, the motion is reversed every few seconds, the transverse induc- 

 tive action only partially approaches its limit. On this account we should be pre- 

 pared to expect, then, that even if the transverse induction were as great as the 

 longitudinal, when time for full development of the induction was allowed, it 

 would be in some degree in defect in the case of the vibrating magnet. 



In the years 1873 and 1874, long before these views on the subject of induction 

 first occurred to the writer, he had had made in Bombay a useful comparison of 

 two Kew umfilar magnetometers, by means of practically contemporaneous obser- 

 vations. The result was to show a persistent difference in the values of the hori- 

 zontal force yielded by the two instruments, far exceeding any probable errors of 



