502 Prof. Fleming and Mr. Clinton on the 



The discrepancies between parts of Tables I. and II. are to 

 be accounted for by the adoption of a different arrangement 

 of connecting wires in the two sets of experiments. The way 

 in which allowance should be made for the capacity of the 

 leads, when this is not very small compared with the capacity 

 to be measured, is at present under investigation, and the 

 above Table can only be taken, therefore, as showing that the 

 capacity of two vertical and parallel wires is not twice that 

 of one wire, unless they are at a distance apart of nearly one- 

 third of their length *. 



Similar experiments were made with two zinc cylinders, 

 the diameter of each cylinder being 1^ inches, and the length 

 of each cylinder 9 feet 1^ inches, the capacity of each cylinder 

 taken alone in free space is taken as unity. If these two 

 cylinders were suspended one foot apart in the centre of the 

 large Laboratory above described, and hung vertically, their 

 total capacity only amounted to 1*46 times that of either 

 cylinder taken alone. A further experiment was made with 

 these cylinders. One of them was cut longitudinally and 

 rolled out into a flat strip, and suspended in the same position 

 vertically, the capacity of the original cylinder being taken as 

 unity. The capacity of the sheet when, rolled out into a strip 

 was found to be 10 per cent, greater than in its original 

 form. 



With the same appliances, a series of experiments was 

 made in the Pender Laboratory on the measurement of the 

 capacity of vertical wires of different diameters. Seven wires 

 of different diameters were suspended vertically by means of 

 silk strings from the ceiling of the Laboratory, and the 

 capacity of each wire was measured separately, a correction 

 being made for the capacity of the terminal, and of course 

 also for the fine w r ire or lead connecting the wire under 

 experiment with the commutator. The lengths of the wires 

 were about 12 feet in all cases. The following Table shows 

 the result of the measurements, the capacity of the wires 

 being given in micro-microfarads (M.M.Fds.), the fourth 

 column showing the capacity as calculated from the formula 



Capacity (in electrostatic units) = — -= jr where / is the 



r J v 2 log e l/r 



length of wire and r is the semi-diameter in centimetres. 



* It is clear that the capacity of the object measured, as it would be 

 free in space, is not obtained exactly by deducting that of the leading 

 wire alone from that of the leading wire and object when connected. 

 The assumption that it is so is only a first approximation to the truth. 



