ELECTROMAGNETIC UNIT OF ELECTRICITY TO THE ELECTROSTATIC UNIT. 587 
upon the top of these a cylindrical ring EL, similar to the ring at the bottom. Another 
brass cylinder, ABCD, made in three pieces, two rings somewhat similar in height to 
the rings HP, EL, and a long middle piece of the same length as the cylinder FGMN, 
is then fitted over the other cylinders, the bottom ring fitting into the outer groove 
in the ebonite board ; the internal diameter of this cylinder is about 25 cm. 
Fig. 1. 
The cylinders are made co-axial by means of three pieces of ebonite worked down 
to the same thickness (the difference between the radii of the cylinders) pushed by 
rods attached to them down between the cylinders, the cylinders are adjusted until 
these three pieces of ebonite arranged symmetrically round the cylinder are each just 
in contact with the two cylinders; the rods were then removed. The insulation 
between the inner and outer cylinders and between the inner cylinder and its guard 
rings was tested by connecting one of these to earth, and the other to a charged gold 
leaf electroscope ; the condenser was not used unless there was no appreciable loss 
of electricity shown by the electroscope in five minutes. 
Calculation of the Ca'pacity .— The capacity of the system regarded as two co-axial 
cylinders of circular section with a uniform distribution of electricity over them is 
where a is the radius of the outer cylinder, h that of the inner, and I the length of 
the cylinder FGMN. 
Correction for want of coincidence hetiveen the Axes .— It is shown in a paper by 
J. J. Thomson, “ On the Determination of the number of Electrostatic units in the 
Electromagnetic unit of Electricity” (‘Phil. Trans.,’ 1883, p. 714), that if c be the 
small distance between the axes of the cylinders, the capacity is 
