This difference in behaviour of the positive and negative ions was investigated in detail by C. T. R. Wilson[68] in the following way. X rays were made to pass in a narrow beam on either side of a plate _AB_ (Fig. 7) dividing the condensation vessel into two equal parts. The opposite poles of a battery of cells were connected with two parallel plates _C_ and _D_, placed symmetrically with regard to _A_. The middle point of the battery and the plate _A_ were connected with earth. If the plate _C_ is positively charged, the ions in the space _CA_ at a short distance from _A_ are all negative in sign. Those to the right are all positive. It was found that condensation occurred only for the negative ions in _AC_ when _v₂_/_v₁_ = 1·25 but did not occur in _AD_ for the positive ions until _v₂_/_v₁_ = 1·31. Thus the negative acts more readily than the positive ion as a centre of condensation. The greater effect of the negative ion in causing condensation has been suggested as an explanation of the positive charge always observed in the upper atmosphere. The negative ions under certain conditions become centres for the formation of small drops of water and are removed to the earth by the action of gravity, while the positive ions remain suspended. With the apparatus described above, it has been shown that the positive and negative ions are equal in number. If the expansion is large enough to ensure condensation on both ions, the drops formed on the right and left of the vessel in Fig. 7 are equal in number and fall at the same rate, _i.e._ are equal in size. Since the ions are produced in equal numbers from a gas electrically neutral, this experiment shows that the charges on positive and negative ions are equal in value but opposite in sign. =36. Charge carried by an ion.= For a known sudden expansion of a gas saturated with water vapour, the amount of water precipitated on the ions can be calculated readily. The size of the drops can be determined by observing the rate at which the cloud settles under the action of gravity. From Stokes’ equation, the terminal velocity _u_ of a small sphere of radius _r_ and density _d_ falling through a gas of which the coefficient of viscosity is μ is given by 2 _dgr²_ _u_ = -------- 9 μ where _g_ is the acceleration due to gravity. The radius of the drop and consequently the weight of water in each drop can thus be determined. Since the total weight of water precipitated is known, the number of drops present is obtained at once. This method has been used by J. J. Thomson[69] to determine the charge carried by an ion. If the expansion exceeds the value 1·31, both positive and negative ions become centres of condensation. From the rate of fall it can be shown that approximately the drops are all of the same size.