618 PROF. J. .T, THOMSON AND MR. G. F. C. SEARLE OX THE RATIO OF THE 
J I - L - 
, ri _ 7 I (y + /3) {b + j + cc) 
i/3 K , - 7^ -1 
1 ^(^ + « + 7)/^i 
or with sufficient accuracy for our purpose 
7 r _ 7 (7 + «) 1 
b^ \ /3 (6 + a + 7) J ’ 
tluis, if we wish to use the formula nC = y/^/3 we must diminish y in the ratio 
^ + '^) to J. The amount by which y is to be diminished is given in 
/ 3 {b + cc + y) ^ & 
column 15. 
Column 16. “Correction to legal ohms.” The values of b and yS given in columns 
( 5 ) and (6) are already expressed in terms of legal ohms, but the values of y are 
expressed in terms of the resistances in the Wheatstone bridge box, which are greater 
than legal ohms in the proportion I'OOll to 1. We have then to add 3’04 ohms to y 
in the first and tlurd sets of experiments, and 3'64 ohms in the second. 
Column 17. “ Correction for the difference of potential between the middle cylinder 
and the guard ring.” By equation (6), page 9, if the difference of potential between 
the outer cylinder and the guard ring is less by SV than that between the outer cylinder 
and the inner cylinder, the capacity is greater than it would be if SV = 0 in the ratio 
I + 
BYh H 
Y 1 |c 
2, 4r 
7 
TT ' li'6 
to 1 
where V is the difference of potential between the cylinders, t the thickness of the 
guard ring, 2c the thickness of the pieces of ebonite, and h the distance between the 
cylinders. 
Since t = \,h= 1, c = T45 and SV/V = — bj\^ot. + /3 if --ZL ' Zjj capacity is 
less than it would be if the guard ring and the cylinder were at the same potential in 
the ratio of 
1 
7'5 by 
G1 «7 + /3 (+ a + 7) 
SO that in order to get the corresponding value of y when the guard ring and cylinder 
are at the same potential, we must add to y 
^5_ by- 
61 a7 + /3 (6 + a +7) ’ 
and it is this correction whicli is given in column 17. 
