60 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



(2) The current due to the ionization of the room air near the fog 

 chamber and on the outside of it, due to gamma rays. This is made a 

 minimum by allowing the thin wire communicating with the electro- 

 meter to run axially away from the fog chamber, for the gamma rays, 

 in spite of their penetrating powers, are quickly reduced by distance. 

 The current is found in the presence of radium within the axial tube by 

 leaving all adjustments identically in place, but breaking the metallic 

 connection between the aluminum core and the electroscope, etc., by a 

 hard rubber insulator. If an auxiliary condenser is used, the measure- 

 ment (i) must be made without it, as otherwise its leak would be counted 

 twice. Fortunately the conduction current is quite negligible. 



(3) The current due to ionization within the fog chamber. This is 

 found by deducting from the total current found on connecting the 

 charged aluminum core and the electrometer the two preceding currents. 



50. Auxiliary condenser. To vary the experiments to the extent 

 that different speeds of leakage may be obtained, as well as to find the 

 capacities of the electrometer and fog chamber, an auxiliary condenser 

 must be inserted as a part of the electroscope. This condenser consisted 

 in the present experiments of two plates of brass, having an area of 315 

 sq. cm., and usually kept at a distance of 0.32 cm. apart by outrigged 

 feet of hard rubber, which stood on a plate of glass. By putting small 

 glass plates under these feet the capacity could be varied at pleasure. 

 The usual equation was corrected by aid of the factor 



i + ( d + din 1 6 v 7 J^ (d + d) /d 2 + din (d + <5) / #) / v 7 !^ 



where a is the area, d the distance apart, and d the thickness of the plate 

 of the auxiliary condenser. Naturally a guard ring condenser would 

 have been preferable for standardization, but none was at hand. 



To determine the very small capacity C of the electroscope-fog-cham- 

 ber, two successive full charges from the lighting circuit, at a potential V = 

 250 volts, were in turn imparted from C to the auxiliary condenser of 

 capacity C'. If V" be the potential observed after these two charges and 

 S=V"j (V-V"), C = C / 5/(i + \/i"+5). It is curious that this method 

 of successive charges leads to involved cubic, quartic, quintic equations, 

 etc., which follow no simple rule. The ratios R of the potentials, after 

 four and after two charges R = V " /V is, however, still available 



for if r = C'/C,thenr=(R-i)i + \/i + (2-R)/(R-i /(2-R). Apart 



from these complications, the large deflections obtainable after many 

 successive charges would, in the absence of conduction leakage in the 

 condensers, make this method very satisfactory. 



In the definite measurements, however, almost the whole capacity 

 may be placed in the auxiliary condenser, so that the capacities of the 



