404 



NA TURE 



[August 23, 1900 



perhaps when made, that the sensitiveness of the apparatus is 

 increased by reducing its dimensions. He therefore decreased 

 the scale as far as was consistent with exact measurement of the 

 parts of the apparatus, using a torsion rod, itself a mirror, only 

 2" long, gold balls, m m, only J" in diameter, and attracting 

 lead masses, M M, only 4J" in diameter. The force to be 

 measured was less than 1/5 x lo® grain. 



The exactness of his work was increased by using as suspend- 

 ing wire one of his quartz threads. It would be difficult to 

 over-estimate the service he has rendered in the measurement of 

 small forces by the discovery of the remarkable properties of 

 these threads. 



One of the chief difficulties in the measurement of these small 

 gravitational pulls is the disturbances which are brought about 

 by the air currents, which blow to and fro and up and down 

 inside the apparatus, producing irregular motions in the torsion 

 rod. These, though much reduced, are not reduced in propor- 

 tion to the diminution of the apparatus. 



A very interesting repetition of the Cavendish experiment 



in place of the somewhat untrustworthy metal wire which he 

 used in the work already published. 



Prof. Boys has almost indignantly disclaimed that he was en- 

 gaged on any such purely local experiment as the determination 

 of the mean density of the earth. He was working for the 

 Universe, -seeking the value of G, information which would be 

 as useful on Mars or Jupiter or out in the stellar system as here 

 on the earth. But perhaps we may this evening consent to be 

 more parochial in our ideas, and express the results in terms of 

 the mean density of the earth. In such terms, then, both Boys 

 and Braun find that density 5 "527 times the density of water, 

 agreeing therefore to i in 5000. 



There is another mode of proceeding which may be regarded 

 as the Cavendish experiment turned from a horizontal into a 

 vertical plane, and in which the torsion balance is replaced by 

 the common balance. This method occurred about the .'ame 

 time to the late Prof. V. Jolly and myself. The principle of 

 my own experiment {Phil. Tians., 182, 1891, A, p. 565) will 

 be sufficiently indicated by Fig. 3. A big bullion balance with 

 a 4-foot beam had two lead spheres, ab, each about 5olbs. in 

 weight, hanging from the two ends in place of the usual scale 



Fig. 2. — Boys' apparatus. 



has lately been concluded by Dr. Braun {Denkschriften 

 der Math. Wiss. Classe der Kais. Akad. der Wissenschaften 

 Wien, Ixiv. 1896) at Mariaschein, in Bohemia, in which he has 

 sought to get rid of these disturbing air currents by suspending 

 his torsion rod in a receiver which was nearly exhausted, the 

 pressure being reduced to about ^\^ of an atmosphere. The 

 gales which have been the despair of other workers were thus 

 reduced to such gentle breezes that their effect was hardly 

 noticeable. His apparatus was nearly a mean proportional 

 between that of Cavendish and Boys, his torsion rod being about 

 9" long, the balls weighing 54 gms. — less than two ounces — 

 and the attracting masses either 5 or 9 kgms. His work bears 

 internal evidence of great care and accuracy, and he obtained 

 almost exactly the same result as Prof. Boys. 



Dr. Braun carried on his work far from the usual laboratory 

 facilities, far from workshops, and he had to make much of his 

 apparatus himself. His patience and persistence command our 

 highest admiration. 



I am glad to say that he is now repeating the experiment, 

 using as suspension a quartz fibre supplied to him by Prof. Boys 



Fig. 3. — Common balance experiment (Poynting). 



pans. A large lead sphere, M, i' in diameter and weighing 

 about 350 lbs., was brought first under one hanging weight, then 

 under the other. The pull of the lead sphere acted first on one 

 side alone and then on the other, so that the tilt of the balance 

 beam when the sphere was moved round was due to twice the 

 pull. By means of riders the tilt, and therefore the pull, was 

 measured directly as so much increase in weight. This increase, 

 when the sphere was brought directly under the hanging weight 

 with i' between the centres, was about \ mgm. in a total weight 

 of 20 kgm., or about i in 100,000,000. If, then, a sphere i foot 

 away pulls with i/io* of the earth's pull, the earth being on the 

 average 20,000,000 feet away, it is easy to see that the earth's 

 mass is calculable in terms of the mass of the sphere, and its 

 density is at once deduced. The direct aim of this experiment, 

 then, is not g, but the mass of the earth. 



It is not a little surprising that the balance could be made to 

 indicate such a small increase in weight as i in 100 million. 

 But not only did it indicate, it measured the increase, with 

 variations usually well within i per cent, of the double attrac- 

 tion, or to I in 5000 million of the whole weight, a change in 

 weight which would occur merely if one of the spheres were 

 moved Vu inch nearer the earth's centre. This accuracy is only 



NO. 1608, VOL. 62] 



