298 MR. C. T. R. WILSON ON THE EFFICIENCY AS CONDENSATION 



the apparatus with its centre in the plane of the central plate. The fogs were best 

 seen when the eye was placed sufficiently below the level of the illuminated stratum 

 to receive none of the direct light, but only that scattered by the drops. 



To compare the effects of positive and negative ions, expansions were made with 

 the left-hand plate at a certain positive potential (generally about 1 volt), the right- 

 hand plate being at an equal negative potential and the central plate earthed. The 

 appearance of the fogs on the two sides was noted, the direction of the fields reversed, 

 and the effect of an expansion of the same amount as before again noted. Any effect 

 due to want of symmetry in the apparatus was thus eliminated. 



To secure equality in the electric fields in the two halves of the apparatus, the 

 following arrangement was used. The source of potential (generally two Leclanche 

 cells in series) had its terminals connected by a resistance of 200 ohms. The middle 

 point of this resistance was earthed, and the two extremities were connected through 

 a commutator to the outer plates of the cloud apparatus, the central plate being 

 earthed. 



The correction to be applied to obtain the true values ot vjv l was found to be the 

 same as before ; to the apparent values of v 2 /Vi '03 must be added. The error was, as 

 before, found to be the same at both the points, t^/t^ = 1'25 and i^/Vj = 1'38. 



To obtain an approximation to quantitative comparison between the number ot 

 drops produced in the two halves of the apparatus, measurements were made on the 

 time taken by the upper surface of the clouds on the two sides to sink below the level 

 of the beam of light, which was used to make them visible. The expansion and 

 coasequent cooling on both sides being the same, the same quantity of water is 

 condensed in each half; the quantity available for each drop is thus inversely pro- 

 portional to the number of drops which are formed. Now the radii of the drops on 

 the two sides can be compared by measuring the velocity with which they fall ; a 

 comparison of the rates of fall on the two sides will thus enable us to determine the ratio 

 between the numbers of the drops produced in the two halves of the apparatus, or will 

 at least serve as a test for equality between the numbers. Professor J. J. THOMSON* 

 has in fact used the rate of fall of the drops, formed on the ions as the result of 

 expansion, to determine the number of the ions present, from which he obtains an 

 estimate of the charge carried by each. 



The results of measurements with this double apparatus are given in the tables 

 which follow. After what has already been said in connection with the other 

 apparatus, it is hardly necessary to point out that, when the right-hand plate is 

 connected to the positive terminal of the battery, and the left-hand plate to the 

 negative terminal (the central plate being at zero potential), there will be a great 

 excess of negative ions on the right, and of positive ions on the left. The terms 

 " positive " and " negative " in the tables refer to the sign of the charge carried by the 

 majority of the ions, not to the potentials of the plates ; the corrected values of t>j/v 



are given. 



* ' Phil. Mag.,' loc. cit. 



