156 
Mr PocJclington, The Natural Units 
units per cm., or 5'9 x 10 ~ 3 \/(m/lt 2 ). We can make the same 
remark as in the last case. 
The conductivity for heat of the true metals ranges from ‘08 
for lead to 1 for copper, the mean being ’28. In the new units it 
is 757. 
The specific resistance of the true metals at 0° C. ranges from 
1’5 x 10~ 6 for silver to 9 x 10“ R for platinum, the mean being 
3'6 x 10~ 6 ohm eras. = 4 x 10 -18 C.G.S. = 73 x 10 _R t. 
The intensity of light required to halve the resistance of 
selenium is about equal to that of direct sunlight, i.e. ’04 cal. 
per sec. per sq. cm. = 1*7 x 10 6 ergs/cm. 2 sec. = 20 x 10 -8 m/£ 3 . 
The wave-length of light that is capable of producing chemical 
action has a higher limit at 5’9 x 10~ 5 cms. = T5 x 10 3 1, 
The rotation of the plane of polarization of light on normal 
reflection from the pole of an electromagnet is from ’019 to '013 
(mean *016) minutes of arc per C.G.S. electromagnetic unit of 
magnetization or 48 x 10~ 6 if we use radians. In the new units 
(electromagnetic) this is T 6t\/(l/m). 
The rotation of the plane of polarization of light passing 
through a transparent body placed in a magnetic field is directly 
proportional to the difference in magnetic potentials at the ends 
of its path and inversely to the square of the wave-length. The 
constant which is obtained by multiplying Verdet’s constant by 
the square of the wave-length has dimensions of length divided 
by magnetic force. Its value in electromagnetic C.G.S. units when 
the angle is measured in radians is a maximum for lead borate, 
being 6'0 x 10~ 14 = 5 - 2 x 10 -2 £ 
The ratio of the maximum elongation of iron produced by 
magnetizing it to the original length is 1/720000 = 14 x 10 -6 . 
Being a mere number it has the same value in the new units. 
The susceptibility of diamagnetic substances has its maximum 
value in bismuth, 2‘5 x 10~ 6 c.G.s. electromagnetic units. Since 
this is a mere number, it will have the same value in the new 
units. 
In the case of ferromagnetic substances there is no definite 
susceptibility. The maximum magnetization is found in iron and 
is 1700 electromagnetic C.G.s. = 5 - 0 x 10 ~ 2 ^(m/lt 2 ) electromagnetic 
units. 
The magnetic force required to half-saturate iron is about 
5 C.G.S. which is 1*5 x 10~ 4 in our units. 
The rotation per centimetre of polarised light passing through 
an optically active substance is roughly proportional to the re- 
ciprocal of the square of the wave-length. The constant involved 
is the rotation per cm. multiplied by the square of the wave- 
length. The maximum is found in cinnabar and is 1*9 xl0 _10 cms. 
or 4 - 8 x 10 -3 £. 
