ACOUSTICS AND GRAVITATION. 11 



and 



If the values of AF in (13) and (14) are the same, since 



by equation (9), it follows that equation (i i) again results. 



11. Absolute values. It is next to be determined whether the equations 

 (A) or (B) of 6 are to be used ; in other words, the value of the condenser space 

 in the electrometer is to be found. For this purpose it was convenient to com- 

 pare the data of the latter with the corresponding results of the Elster and 

 Geitl electroscope. There happened to be three of these in the laboratory, all 

 standarized by the aid of storage-cells compared with the Clarke cell, the range 

 of available voltages being of the right order. To obtain corresponding read- 

 ings, it was merely necessary to join the U-tube electrometer and the electro- 

 scope in parallel with the electrophorus, figure 7, and gradually depress the 

 plate p on the micrometer-screw s. 



The results of these comparisons are shown in figure 12, the U-tube reading 

 being reduced to volts by equation (B), 6, and the electroscope readings by 

 the charts. Not much accuracy is to be expected in the individual readings, 

 but the mean results of different instruments are quite trustworthy. The 

 ratio of data (the U-tube numbers being in excess) is for the case of electro- 

 scope No. 1499, 4.3 and 4.2; for No. 1727 it is 4.3, and for No. 1277 (this was the 

 Ebert apparatus for testing atmospheric ionization, not well adapted for the 

 present purposes) about 3.9. The mean of these results is 4.2, so that the data 

 of the U-tube electrometer computed by equation (B) are about 4 times too 

 large. This equation is thus inapplicable and the simpler equation (A) is to 

 be taken. In other words, the thin glass plate floating on and in intimate 

 contact with the mercury acts like a conductor and carries the charge practi- 

 cally at once to its upper face. Thus an equation of the type (A), 6, 



should have been used, and this would make the distance between the lower 

 face of the upper electrode and the upper glass face 60/95 = 0.63 mm - 



Examining the apparatus, it was found that in the long usage to which it 

 had been put the levels had slightly shifted. The distance between the 

 electrode surfaces is difficult to determine without special apparatus. I 

 obtained an estimate by rolling iron wires of different diameters between the 

 faces. In this way, with some patience, 0.07 cm. was obtained. It is suf- 

 ficiently near the datum from electroscope comparisons to prove the case. 

 The equation should, therefore, read 



n volts 



