70 THE ROYAL SOCIETY OF CANADA 
was thus unnecessary,—a great advantage when determing the capacity. 
One division of the microscope scale corresponded to 2-65 volts. 
Capacity. 
It is difficult to determine with accuracy a small capacity, such as 
2 or 3 cm. A method is given by Harms in the Phys. Zeit. (15 Jan., 
1904), and another by Campbell (Phil. Mag., Jan., 1911), but these 
involve, to some extent, connecting wires. 
I have found it possible to obtain the capacity by taking straight 
thin wires, of length 1 and radius r, the capacities of which, away from 
conductors, are given by 
2 log, l/r 
Two such wires, each of length 20 em., having separately a capacity 
of 1-82, when placed end to end, together have a capacity of 3-21. 
When a vertical wire of length 20 em. was placed in contact with the 
axis of the electroscope, it was regarded as approximately one-half of 
a wire of 40 em. length. There are further corrections, such as that 
due to the approach of the wire to the earth-connected upper plane 
of the electroscope. Wires of 20, 30, 40, 50, and 60 cm. were used, of 
diameter -O8 em., and a similar series of diameter -048 cm. The 
electroscope, of unknown capacity C, had its vertical system charged 
to potential V, and when the ne vertical wire touched it, the 
potential fell to V’. Thus 
V=(C + C+ 2)V’ 
where C’ is the capacity of the wire found by calculation (as for half a 
wire of double length), apparently varying approximately inversely as 
the length, and where x is a correction. A large number of simultaneous 
equations were determined from the experiments, and these were 
plotted and solved graphically. A further check on the result was ob- . 
tained by bringing up to the top end of a wire, when in contact with 
the central axis, spheres of known radii. The value of the capacity 
was found to be 2-90 cm., with a possible error of about two per cent. 
Measurements: 
Using the well known relation 
qSe = CV/300t* 
* Rutherford’s “ Radioactivity,’”’ p. 87. 
