Ether for Electricity and Magnetism . 307 



with no contributive action of the ether in the space around 

 them and between them. 



§ 24. Stress in ether, being thus freed from the impossible 

 task of transmitting both electrostatic and magnetic force, is 

 (we may well imagine) quite competent to perform the 

 simpler duty of transmitting magnetic force alone. 



§ 25. Hitherto one seemingly insuperable obstacle against 

 following up this idea to practical realization has been the 

 greatness of the force in many well known cases of magnetic 

 attraction between iron poles, whether due to steel magnets 

 or electromagnets. Considering that in our most delicate 

 experiments in various branches of science, ponderable bodies 

 large and small are observed to be moved freely by forces 

 of less than a thousandth of the heaviness * of a milligram, 

 how can we conceive the ether through which they move to 

 be capable of the stress required for the transmission of force 

 between flat poles of an electromagnet amounting per square 

 centimetre to more than two hundred f times the heaviness 

 of a kilogram ? This difficulty is annulled if we adopt the 

 hypothesis which I have described to the Congres (§ 2 above). 

 We may now suppose the density of ether as great as we 

 please, subject only to the limitation that it must not be so 

 great as to disturb sensibly the proportionality of effective 

 inertia to gravity in different kinds of matter, proved by 

 Newton in his pendulum experiment, for lead, brass, glass, 

 &c, and by his interpretation of Kepler's third law for the 

 different planets of our system. Probably we might safely, 

 if we wished it, assume the density of ether to be as much as 

 10~ 6 . I am content at present, however, to suggest 10 -9 . 

 This, with the velocity of light 300,000 kilometres per second, 

 makes the rigidity (being density x square of velocity) equal 

 to 9.10 11 dynes per square centimetre, which is somewhat 

 greater than the rigidity of steel (7.10 11 ). It is clearly not 

 for want of strength that we need question the competence of 

 ether to transmit magnetic force ! I confess that I now feel 

 hopeful of seeing solved some of the other formidable diffi- 

 culties which meet every effort to explain electric insulation 

 and conduction, and electromagnetic force, and the magnetic 

 force of a steel magnet, by definite mechanical action of ether. 



* I cannot without ambiguity use the simple word " weight " here » 

 because this word means legally a mass, and is practically used more 

 often to signify a mass than the gravitational heaviness of a mass. 



t The most intense magnetic field hitherto measured is, I believe, that 

 of Dubois (see his Report on Magnetism to this Congress) in which he 

 found 76,000 c.G.s. between two small plane end-faces of son; iron poles 

 of a powerful electromagnet. This makes the attraction per square 

 centimetre of either face (76,000) 2 -^-87r, or approximately 23. 10 7 dynes, 

 or 230 kilograms. 



