86 Pi of. Chattock, Miss Walker, and Mr. Dixon on the 



curve prolonged cuts the axis of z, it is clear that 



Pi/Ps= (^i — «o)/(-2— *<>)• 

 Ksr. 2. 



/7AT/J OA 



Provided £ is constant, a real change in V will bring about 

 an alteration in the pressures such that their ratio is still 

 (z 1 — Zo)/(z 2 — Zn) ; whereas if the change is apparent only the 

 ratio will alter. It is thus possible to discriminate between 

 the two kinds of variation of V, and for this purpose it is 

 important to know the value of z . 



The table on page 87 contains the values of z calculated 

 for the various groups of experiments given above, and one 

 or two others. 



z for the embedded ring is measured from the plane 

 dough its centre instead of from 0. 



As the brass and platinum rings are of the same size and 

 shape, the values of z obtained with them should be com- 

 parable with each other, though not with those for turpentine 

 and the embedded ring. If we leave out of account the two 

 latter sets, it will be seen that, apart from the negative dis- 

 charge in hydrogen, the value of z does not vary more than 

 about 2 millimetres on either side of 0'06 centimetre, its 

 mean value. 



That variations in z should occur with a ring which 

 catches any of the wind-pressure is of course intelligible ; 

 for the amount caught will depend on the paths taken by the 

 ions in approaching the ring, and these must depend to some 

 extent on the relation between the current-density and the 

 potential slope in that region, which will vary with the gas as 

 well as with the sign and strength of the current. This is 

 borne out by the way in which the discrepancies between the 

 values of z for air and for carbon dioxide almost vanish 

 when the embedded ring is used ; for there is then no reduc- 

 tion in the force of the wind by the ring. 



In the case of hydrogen, the same argument leads to the 

 conclusion that the large value of z for negative discharge is 

 not du^ to the stoppage of an abnormally large amount of 



