470 Sir Oliver Lodge on a possible means of determining- 



area and of length to be subsequently settled on optical con- 

 siderations ; wind it with wire so that, if possible a thousand 

 turns are piled up on every centimetre of its effective length; 

 keep it so cold, say by liquid air, that it can carry 1000 amperes 

 or 100 c.g.s. units of current ; then n x I = 10 5 c.g.s. units. 

 Wherefore v will be 



1 centimetre per second. 



That is to say in a magnetic field excited by a million 

 ampere-turns per centimetre, the rate of aether flow along 

 the lines of force will be only 3 millimetres per second or 

 7 inches a minute — about a snail's crawl. This is the speed 

 which has to be compared with the velocity of light. 



A considerable length of optical circuit becomes clearly 

 necessary. Let each side of the supposed equilateral triangle 

 constituting our magnetic circuit be a helix 10 metres long, 

 and let the optical arrangements be so perfect that a shift 

 of voVo °^ an interference band can be detected, then the 

 ratio which we have available for comparing the velocities- 

 is the ratio of the total length of the circuit to one-thousandth 

 of a wave-length ; that is to say, 



30 metres 



7TT — n 1 — or 5 x 10 10 . 



b tenth-metres 



The speeds to be compared are 



3x1010 inn 



— r y or 10. 



Hence by taking advantage of the opposite transits of each 

 half of the light beam, and taking advantage also of reversal 

 of the magnetic field, the observation seems just barely 

 feasible. 



If the light were sent round a square circuit two or three- 

 times, as was done in my iEther experiment (Phil. Trans, 

 vol. 184, A, 1893, page 757), it would seem possible to make 

 a sort of measurement of the speed, and therefore of the 

 setherial density, even in the extreme case of its being as- 

 high as the estimate here suggested as a maximum. The 

 density can hardly be greater : if it is less, the determination 

 is proportionately easier. I expect it to be rather less, but 

 not much : I think the order of magnitude is probably right. 

 If there is no shift at all, and if the absence of drift can thus 

 be certainly established, we shall learn that something is 

 wrong with the hypothesis of setherial flow along magnetic 

 lines of force ; for it is on that hypothesis that the whole 

 suggested experiment has been founded. 



