404 TRANSACTIONS OF SECTION A. “ee ne 
9. The Minimum Quaxdtity of Light discoverable by means of Selenium. 
By Dr. E. E. Fournter p’ ABE. 
This paper dealt with the behaviour of selenium under very faint illumina- 
tions. The changes in conductivity resemble phenomena of ionisation and recom- 
bination, inasmuch as the light-action curve satisfies the equation dK /dt=I—CK’, 
where I is the illumination, C the coefficient of recovery on recombination, and 
K the additional conductivity imparted to selenium by the action of light. The 
initial light-action is a linear function of the time, so that the action observable 
in a given time is proportional to the ‘exposure’ if the latter is short. The 
final additional conductivity is proportional to the square root of the illumination. 
This offers a means of discovering very faint illuminations. A current of the 
order of 10° ampere is obtainable-by the unaided illumination produced by 
Venus, and it should be possible to discover invisible stars by means of a 
sensitive galvanometer without any optical aid. Attempts to do this were de- 
‘scribed, and the bearing of the results on the theory of quanta was discussed. 
(Some new applications of the properties of selenium were demonstrated at 
the University buildings.) 
10. Discussion on Professor W. H. Bragg’s Paper on Crystals and 
X-Rays. 
11. A New Process for Enlarging Pholographs. By A. J. Lorna. 
12. A Magnetic Susceptibility Meter. By W. H. F. Murpoct. 
The author uses a unipolar method of testing, with the addition of a circular 
coil in series with the magnetising solenoid. This coil acts upon the magneto- 
meter needle which is lying in a neutral field of force, so that the tangent of the 
resulting deflection due to the magnetic material plus coil is strictly propor- 
tional to magnetic susceptibility. 
A mirror may be used to read the deflections, and in this case susceptibility 
= deflection x constant. 
It follows that such an instrument with tangent scale and pointer can be 
graduated so as to read directly the value of the susceptibility coefficient in 
C.G.S. units. To fix the value of H the current is measured and the values 
multiplied by a constant. 
The theory of the instrument was given, and an example of its use for testing 
iron, together with a diagram of connections and curve of results. 
The various corrections were also briefly discussed. 
13. The Sensitiveness of the Human Skin as a Detector of Low Vollage 
Alternating E.M.F.1 By Professor H. Sransrietp, D.Sc. 
An alternating difference of potential of 40 volts, with a frequency of 
50 cycles per second, between a piece of metal and the human body, is sufficient 
to produce a vibration which can be felt when the back of the hand is lightly 
rubbed against the metal. If the metal is connected to a live wire, the observer 
should be well msulated from the earth. 
‘he alternating EK.M.F. acting across the thin badly-conducting surface layer 
of the skin produces a rapid variation of the frictional force. If the metal is 
allowed to rub against the ear, instead of the hand, the vibration is heard as 
a inusical note. 
‘The human skin may be imitated for this purpose by gilding a rounded lump 
ot jelly with gold leaf and then covering it with a piece of thin silk. The 
imitation is not, however, as sensitive as the original, 
1 Printed in full in the Hlectrician, September 26, 1913. s 
