1889.] 



On the Cavendish Experiment. 



263 



In my apparatus it is simply necessary to multiply the cube of the 

 diameter of the large balls by the ordinate of the curve, to find on 

 the same scale the value of the deflection. This requires that the 

 large balls should be made of material of the same deusity in the two 

 cases, and that the periods should be the same. Now the diameter of 

 the large ball in the new apparatus is 6"4 times the length of the 

 beam, and so the comparative value of the deflecting force is 0'0425 x 

 6"4 3 = ll'l, a figure which is 18*7 times as great as the figure found 

 for the apparatus of Cavendish. If then the large balls have the 

 same density as those used by Cavendish and the period of oscillation 

 is the same, the angular deflection will be 18' 7 times as great. 



Having now found that with apparatus no bigger than an ordinary 

 galvanometer it should be possible to make an instrument far more 

 sensitive than the large apparatus in use heretofore, it is necessary to 

 show that in practice such a piece of apparatus will practically work, 

 and that it is not liable to be disturbed by the causes which in large 

 apparatus have been found to give so much trouble. 



I have made two instruments of which I shall only describe the 

 second, as that is better than the first both in design and in its 

 behaviour. 



The construction of this is made clear by fig. 6. To a brass base 

 provided with levelling screws is fixed the vertical brass tube, t, 

 which forms the chamber in which the small masses a, b are sus- 

 pended by a quartz fibre from the pin at the upper end. These little 

 masses are cylinders of pure lead 11*3 mm. long and 3 mm. in 

 diameter, and the vertical distance between their centres is 50*8 mm. 

 They are held by light brass arms to a very light taper tube of glass, 

 so that their axes are 6'5 mm. from the axis of motion. The mirror 

 m, which is 12' 7 mm. in diameter, plane and of unusual accuracy, is 

 fastened to the upper end of the glass tube by the smallest quantity 

 of shellac varnish. Both the mirror and the plate-glass window 

 which covers an opening in the tube were examined and afterwards 

 fixed with the refracting edge of each horizontal, so that the slight 

 but very evident want of parallelism between their faces should not 

 interfere with the definition of the divisions of the scale. The large 

 masses M, M are two cylinders of lead 50*8 mm. in diameter, and of 

 the same length. They are fastened by screws to the inside of a 

 brass tube, the outline of which is dotted in the figure, which rests on 

 the turned shoulder of the base so that it may be twisted without shake 

 through any angle. Stops (not shown in the figure) are screwed to 

 the base, so that the actual angle turned through shall be that which 

 produces the maximum deflection. A brass lid made in two halves 

 covers in the outer tube and serves to maintain a very perfect uni- 

 formity of temperature in the inner tube. Neither the masses M, M, nor 

 the lid touch the inner tube. The period of oscillation is 80 seconds. 



