C. Bar us — Thomson's Constant. 325 



condenser except when not in use and by sheathing it with an 

 annular air space beyond the condenser. It is found experi- 

 mentally, by direct measurement in the absence of radium. 

 (2) The current resulting from 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 electrometer to run axially away 

 from the fog chamber ; for the gamma rays, in spite of their 

 penetrating power, are quickly reduced by distance. This 

 current is found in the presence of radium within the axial 

 tube, by leaving all adjustments identically in place, but break- 

 ing the metallic connection between the aluminum core and 

 the electroscope, etc., by a hard rubber insulator. If an 

 auxiliary condenser is used, the measurement (1) must be 

 made without it, as otherwise its leak would be counted twice. 

 Fortunately the conduction current is relatively quite negli- 

 gible. (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 elec- 

 trometer, the two preceding currents. 



3. 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 ctn , 

 and usually kept at a distance of •32 cm apart by outrigged 

 feet of hard rubber, wmich stood on a plate of glass. By put- 

 ting small glass plates under these feet, this capacity could be 

 varied at pleasure. The usual equation was corrected by aid 

 of the factor 



1 + (d+dln 16 VaTT (d + 0)/d 2 + 6 In (d + 6)/6)/Vair, 



where a is the area, d the distance apart and 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-chamber, two successive full charges from the lighting 

 circuit, at a potential F=250 volts, were in turn imparted 

 from C to the auxiliary of capacity C . If V" be the potential 

 observed after these two charges and jS = V" / ( V — Y"J, 

 C= C S/{1+ v/1 + 8). It is curious that this method of 

 successive charges leads to complicated cubic, quartic, quintic 

 equations, etc., which follow no simple rule. The ratios R of 

 the potentials after four and after two charges B, — Y"" / Y" 

 is however still available. Apart from these complications, 



