18 
MR. E. RUTHERFORD OR A MAGNETIC DETECTOR OP 
If the value of [x, the permeability of the specimens of soft iron, be calculated from 
the formula 
R' = y 
it will be found that the value varies with the diameter of the wire. 
Diameter. 
/'■ 
centims. 
■02.5 
3-5 
•042 
9-4 
•094 
18 
•295 
53 
Ct/ry 
9 (4). 
yf. 
X 
1 
/ 
/ 
• 
r 
A 
D/dm 
9ter c 
1 
V 
The above curve (Curve 4) shows the relation between the diameter of the wire 
and the permeability for the discharge. It will be seen that jx varies approximately 
as the radius of the ware. 
The very small value of the permeability for fine wires is to be expected ivhen we 
consider the very large currents that pass through the wire, and the consequent 
large value of the magnetic force that acts at the surface of the iron. 
The maximum current of discharge, assuming the damping to be small, is given by 
y = rCV^, where C is capacity and Vq tlie potential to which condenser is charged. 
In the above experiments the air-break was about inch, and the diflerence of 
potential about 10,000 volts, and since p = 5•10*’, C — 4000, the maximum current 
was about 222 amperes. 
The magnetic force at the surface of the ware of radius r, which conveys the 
current, is given by 
H = = 3552 C.G.S. units, if ?’ = *0125 centim. 
r 
