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LXIII. A new Theory of Terrestrial Magnetism. 

 By Professors John Perry and W. E. Ayrton*. 



IN the autumn of 1876, while experimenting on magnetic 

 transparency, we designed an apparatus for testing whether 

 a moving body having a definite electric charge would, like a 

 current, deflect a magnet. While waiting for the conclusion 

 of the rains, and the advent of the very dry season which ac- 

 companies a Japanese winter, in order to try our instrument 

 in conjunction with an ordinary plate-glass electrical machine, 

 we received the account, published in the Philosophical Maga- 

 zine for September 1876, of the experiments just performed 

 by Mr. Rowland in the laboratory of Professor Helmholtz, 

 by which it had been conclusively shown that a charge of 

 electricity mechanically moved had the properties of an ordi- 

 nary electric current as far as the deflection of a magnet was 

 concerned f. 



Until this point was settled, it was unlikely that attention 

 would be directed to the electromagnetic effects that might 

 arise from the rotation of a charged body like the earth. 

 Shortly, however, after the execution of the experiments re- 

 ferred to we attempted (as described in our paper " On Pain 

 Clouds and Atmospheric Electricity," which appeared in the 

 Philosophical Magazine for March 1878) the solution of a new 

 theory of terrestrial magnetism. This problem we have 

 attacked in a variety of ways ; and the following solution, to 

 which we have at length been led, we beg to offer for the ac- 

 ceptance of the Physical Society. 



The points near the surface of the earth have different linear 

 velocities from those in the interior (although all the points 

 have the same angular velocity of rotation round the earth's 

 axis) ; therefore, if the earth had an initial electrical charge, 

 residing of course, in accordance with the well-known electrical 

 law, on its surface, the electrified particles would have veloci- 

 ties relative to the remainder ; hence, as a direct consequence 

 of the results of the experiments published by Professor Helm- 

 holtz, the interior of the earth would be a magnetic field, quite 



* Communicated by the Physical Society, having been read at the 

 Meeting on March 8. 



t Additional confirmation has recently been given on this subject by 

 the experiments described by Mr. Crookes in his paper on the " Illumina- 

 tion of the Lines of Molecular Pressure &c," since he has shown that the 

 stream of particles which is shot off frorn the negative terminal in a very 

 perfect vacuum, and which produces the green phosphorescence, carries 

 electricity with it and is deflected by a magnet. This may be regarded 

 as a sort of converse experiment, since it proves that a magnet deflects a 

 moving charge of electricity. 



