368 
DR, J. HOPKINSON ON THE ELECTROSTATIC 
working was attained by approximating, so far as my instruments admitted, to tbe 
methods of Mr. Gordon. B and D were, as before, connected to the quadrants, C was 
connected to the interior of the jar and to one pole of an ordinary induction coil ; 
A and E to the case of the instrument and to the other pole of the induction coil. The 
plate E was adjusted till the working of the coil caused no deflection of the image on 
the scale. In each case the plate examined was placed approximately half-way between 
D and E. The following table gives results of a plate of double extra-dense flint 
24*75 millims. thick and 235 millims. diameter, and on a plate of brass 3’5 millims, 
thick and 242 millims. diameter. 
Column I. gives the air-space between the plates AB, BC, or CD. 
Column II. the air-space DE (cq) when no dielectric plate was present. 
Column III. the distance DE (cq) when a dielectric was introduced. 
Column IV. the value of tbe difference 6 —(cq—cq), h being the thickness of the 
plate, which ought to be constant for each plate. 
Column Y. the specific inductive capacity = 7 . ^ 
1 1 17 i-Cq-fq) 
Double extra dense flint, 24‘75 m.m. thick. 
I. 
II. 
III. 
IV. 
V. 
5 
5 i 
27 
3 
8”25 
8 
H 
30i 
93 
^4 
9-0 
12 
iif 
3 If 
4f 
5-21 
18 
21 
37i 
H 
2-91 
25 
321 
48f 
131 
1-83 
32 
441 
491 
19f 
1-25 
True value of K = 9’896. 
Brass plate, 3'5 m.m. thick. 
I. 
II. 
III. 
IV. 
V. 
5 
4-5 
6-75 
1-25 
2-8 
8 
8-0 
6-25 
5-25 
0-66 
12 
11-25 
io-o 
4-75 
0-73 
32 
44'5 
16-5 
31-5 
o-ii 
True value of K= co ty . 
Inspection of the column IY. shows how impossible it is to attribute the variations 
of K to any mere error of observation even with the roughest appliances. Column Y, 
demands no comment. 
