88 0. Barus — Standardization of the Fog Chamber. 



behavior is puzzling and needs farther explanation. It is 

 clearly of grave importance.* 



These difficulties are inherent in the phenomenon and 

 merely exhibited by the fog chamber. The latter has the 

 great advantage that enormous nucleations, like millions per 

 cubic centimeter, are not excluded. Under these circumstances 

 the coronas alone are available for finding the nucleation 

 inasmuch as nearly all the particles evaporate before subsiding. 



3. Preliminary Data. — To test the efficiency of the fog 

 chamber it is necessary to make a preliminary measurement 

 of Thomson's e. Let the radii of the electrical condenser be 

 R x and i? 2 and its length I • let (electrostatic units) be the 

 capacity of the electric system (condenser and electrometer,, 

 together with such auxiliary capacity as may be inserted to 

 get a leakage of proper value) ; let U be the combined velocity 

 of the ions in afield of one volt per cm., Fthe voltage and T^the 

 change of voltage per second ; finally let N be the nucleation 

 given for the identical condenser core when placed in the fog 

 chamber. Then for the cylindrical condenser (if natural loga- 

 rithms be taken) 



e = ( C VlnR l I i? 2 ) / (600 ttIU VJV). 



If V is small enough to keep V/ V constant, the curves 

 show this at once. ' Rough tests using the old data of my fog 

 chamber led to values of about as follows, when C — 130cm ; 

 U = 3'2cm/sec. ; Uthe fraction of a volt (less than one-half) 

 and the field - T volts/cm. 



V/ V— '050 JV= 185,000 6X 10 16 = 3-7 electros, units 

 V/V=-0Q0 ^=210,000 <?X10 10 :r=4-3 " 



The irregularities here are in the electrometer, as the con- 

 necting wires were not at the time surrounded by earthed 

 pipes. On enclosing these, there was less irregularity, though 

 the current was not quite proportioned to. the voltage even for 

 the low values of the latter. In a field of volt/cm, Y—'7, 

 F/F=-040, iT- 150,000 10 10 X^ = 3-8 were obtained. 

 When the condenser was disconnected there was no leakage, 

 showing the piping to be nearly free from such currents as 

 might result from irregular penetration of the gamma rays. 



*As the radium is moved from the glass end to the brass end of the long 

 fog chamber, the corona of maximum diameter (maximum nucleation) 

 moves at a greater rate in the same direction ; so that with radium in the 

 middle, the maximum nucleation is already at the brass end, and then dimin- 

 ishes. It is surprising to notice the largest corona in the middle of the 

 chamber, before the radium gets there. It is also possible to produce mini- 

 mum of nucleation in the middle. The maximum rarely coincides with the 

 position of the radium. Beta and gamma rays are alone active in these 

 curious conflicts of primary and secondary streams. 



