286 



Professor John H. Poynting 



[Feb. 23, 



of force, and the lines are deflected. If the sphere be diamagnetic, of 

 water, or copper, or bismuth, the permeability being less than that of 

 air, there is an opposite effect, as in Fig. 6, and the field is weakened 

 at the end of a diameter parallel to the lines of force, and again the 

 lines are deflected. Similarly, a dielectric body placed in an electric 

 field gathers in the lines of force, and makes the field where the lines 

 enter and leave stronger than it was before. 



If we enclose a magnet in a hollow box of soft iron placed in a 

 magnetic field, the lines of force are gathered into the iron and largely 

 cleared away from the inside cavity, so that the magnet is screened 

 from external action. 



Now, common experience might lead us at once to say that there 

 is no very considerable effect of this kind with gravitation. The 

 evidence of ordinary weighings may, perhaps, be rejected, inasmuch 

 as both sides will be equally affected as the balance is commonly 

 used. But a spring balance should show if there is any large effect 

 when used in different positions above different media, or in different 



Fig. 5. — Paramagnetic sphere placed 

 in a previously straight field. 



Fig. 6. — Diamagnetic sphere placed 

 in a previously straight field. 



enclosures. And the ordinary balance is used in certain experiments 

 in which one weight is suspended beneath the balance case, and sur- 

 rounded, perhaps, by a metal case, or perhaps, by a water-bath. Yet 

 no appreciable variation of weigbt on that account has yet been noted. 

 Nor does the direction of the vertical change rapidly from place to 

 place, as it would with varying permeability of the ground below. 

 But perhaps the agreement of pendulum results, whatever the 

 block on which the pendulum is placed, and whatever the case in 

 which it is contained, gives the best evidence that there is no great 

 gathering in, or opening out of the lines of the earth's force by 

 different media. 



Still, a direct experiment on the attraction between two masses 

 with different media interposed was well worthy of trial, and such an 

 experiment has lately been carried out in America by Messrs. Austin 

 and Thwing.* The effect to be looked for will be understood from 

 Fig. 7. If a medium more permeable to gravitation is interposed 



Physical Review, v. 1897, p. 294. 



