506 Prof. Fleming and Mr. Clinton on the 



long and d centimetres in diameter, and at a height h centi- 

 metres above earth, is given in micro-microfarads by the 

 formula 



0= f 5^" (MJMLFds.) 



logio ± h l d 

 or very nearly by 



C= M l . ^(M.M.Fds.). 



To test this last formula, an experiment was made in the 

 open air with a long wire held parallel to the earth's surface, 

 on insulators at a height of about six feet above the ground. 

 The length of this wire was 500 feet or 15,240 cms. The 

 height of the wire above the ground was 6 feet or 183 cms., 

 and the diameter of the wire was '0645 inch, or 0*164 cm. 

 The observed value of the capacity of this wire in situ was 

 1081 micro-microfarads. The value of the capacity calculated 

 from the formula 



C (in M.M.Fds.) = 1 Q ' 24 }f^ 



is nearly 1000 micro-microfarads, the difference between this 

 calculated and the observed value being about 8 per cent. 



In the same manner the capacity of a vertical wire was 

 measured in the open air, and compared with the theoretical 

 value as given by calculation. In this case the length of 

 the wire was 111 feet, or 3385 cms., and the diameter of the 

 wire was 0*085 inch, or 0*215 cm. The observed value of 

 this wire when suspended vertically in the air with the 

 bottom end about six feet from the ground, is 205 micro- 

 microfarads. The value calculated by the formula 



I 



C (in M.M.Fds.) = 



4-1454 log 10 2tfd 

 is 181 micro-microfarads. The observed value is again 

 greater than the calculated value by about 10 per cent. 



It will thus be seen that in all these cases the observed 

 value of the capacity of the wire, whether vertical or hori- 

 zontal, appears to come out, roughly speaking, about 10 per 

 cent, greater than the calculated value. Approximately, the 

 same difference was found in the case of the capacity of a 

 zinc disk suspended in the Pender Laboratory. The disk 

 was made of sheet zinc circular in form, and 60 inches in 

 diameter. The calculated capacity of this disk in free space 

 is 48*1 electrostatic units, or 53*44 micro-microfarads*. 



* The capacity of a thin circular disk when insulated in infinite space 

 is numerically equal to d/n in electrostatic units, where d is the diameter 

 of the disk in centimetres. 



