Electromagnetic to the Electrostatic Unit. 307 



mean taken, it is thought the temperature is true to within o, l 

 and this corresponds in the case of the larger ball to about '07 g. 

 I think the values of the radii given above are true to within 

 two or three thousandths of a millimeter. 



Shell. 



The weighings of water contained by the shell were made by 

 replacement. The shell was sealed about the junction of its 

 two halves with white paint, filled with distilled water and 

 allowed to stand to absorb any air bubbles which might have 

 escaped the brushing with a wire which was given the inner 

 surface after filling. The condenser was placed on the plat- 

 form of the scales, approximately counterpoised and then ac- 

 curately balanced by adding weights to the platform ; about 

 100 cc. of water was then withdrawn, temperature taken, shell 

 refilled, (the space around the opening being thoroughly raked 

 with a wire to prevent error from small air bubbles which 

 tended to lodge there), and weights again added to balance. 

 The following weighings were thus made : 



130-7 g. 130-5 131-2 131-3 



132-0 132-0 131-7 131-4 Mean = 131-4 g. 



131-6 131-4 Mean temp. 18° -4 



The condenser being emptied and carefully dried required 

 the following weights to balance the counterpoise. 



8650-3, 8650-8, 8650-9, 8650*9; mean 8650-7 



8650-7 — 131-4 = 8519-3 = W't of water at 18°-4 in air. 

 8531-8= " " 4°-0 " 



Correction for displaced air 9-1 



8540-9 g.= " " " in vacuo 



= volume of the shell in cubic centimeters. This makes the 

 radius r = 12*6805 cm. It seems reasonable to suppose that 

 the number 8540*9 is true to within less than a gram. This 

 would make the error in r less than -0005 cm. 



These values of the vadii are confirmed by the following 

 direct measurements made on a dividing engine, using calipers 

 and a standard meter bar by Bartels and Diederichs, Gottin- 

 gen, whose length is accurately known. Three mutually per- 

 pendicular diameters of the shell were found to be 25*357, 

 25*360, 25*358 cm. Mean = 25*358, giving r = 12*679, a very 

 close agreement in view of the difficulty of setting the calipers. 

 More accurate measurements on the balls were obtained. 



Ball A. Following are twelve diameters. 



