716 
MR. J. J. THOMSON ON THE NUMBER OE ELECTROSTATIC 
much more sensitive than the one just described, as the Leyden-jar used in 
Maxwell’s method can easily be raised by an electrophorus to a very high potential. 
The new condenser consisted of several co-axial tubes represented in section in fig. 5, 
The alternate tubes were connected together, and the two series connected with 
opposite plates of a very fine plate condenser, which was very kindly lent to me by 
the Rev. Coutts Trotter, Fellow of Trinity College. 
Fig. 5. 
A rough adjustment could be made by altering the number of tubes connected 
together, while the fine adjustment was effected by altering the distance between the 
plates of the plate condenser by means of a finely cut screw. The equality of this 
condenser and the guard ring condenser was tested by the method given in § 229 of 
Maxwell’s ‘ Electricity and Magnetism,’ using a key which was very kindly lent 
to me by Dr. John Hopkinson, F.R.S., and which had been used by him for a similar 
purpose. I quote the description given of it by him in his paper on the “ Electro¬ 
static Capacity of Glass,” Part II., p. 360, Phil. Trans., Part II., 1881 :— 
“ Fig. 3. 
