ELECTROMAGNETIC UNIT OF ELECTRICITY TO THE ELECTROSTATIC UNIT. 597 
The corresponding ends of the measured diameters were found to be almost exactly 
on the same generating line, so that though the cylinder is slightly elliptical and 
conical, it is free from anything of the nature of helicality. 
Measurement of the Internal Diameter of the Outer Cylinder. 
This was found to be a good deal more troublesome than the measurement of the 
externa] diameter of the inner cylinder, the plan finally adopted was to fix two pieces 
of hardened steel to the ends of the jaws of the sliding calipers, thus 
One side of each piece was polished, and the end was then ground and polished on 
a fine oilstone so as to form a good edge with the polished face. The shape of the 
end was semi-cirenlar. In this way the edges made contact with the cylinder, and 
the cross wires (one of which was set carefully perpendicular to the line of travel of 
the microscopes) could be easily focussed on to the end of the steel. 
It was not found practicable to determine exactly when contact was made in the 
same way as vas done for the inner cylinder, since when the calipers were set to 
nearly the size of the cylinder scarcely any movement was possible. The sharp edges 
were also an impediment to the motion. We found, however, that by insulating one 
of the steel contact pieces we could determine accurately by the aid of a telephone 
when contact was made. As the cylinder wms found to be nearly circular, and the 
formula for a slightly elliptical cylinder outside a circular one indicates that the 
lengths of two diameters at right angles to each other are recpiired, two such diameters 
were measured. The followino; are the details of the measurements, each of the 
numbers being the mean of four observations:— 
Top end. 
Heametek a. 
Calipers. 
25-4 cm. 
Left-hand screw. 
Right-hand screw. 
Left-hand screw. 
Right-hand screw. 
(1) 
I-;I8:2I 
IT 721 
1-3859 
1-1798 
(2) 
i-;3;H9 
1-2U60 
l-:3255 
1-2324 
(3) 
IT 822 
1-344.5 
1-1462 
1-3993 
