Evening star., 1919-09-21

THE SPEAKING TUBE. THE BUsl
possible to construct the apparatus
in its most convenient form. There
appears also to be room for consider
able improvement in, the selenium
cells which have been used as receiv-
ers and it Is quite probable that the
range can be increased to several
miles,"
The great advantage of the sun
phone is that messages can be tapped
only by any one getting in the way
of the beam of light. This would be
obvious to those using the instrument
at once. AS it could be so easily de-
tected, it would not be worth while
attempting it. Wireless telephone
messages are easily tapped. 1t is
only a matter. Of finding the right
key. You can, also join wireless"
but you cannot join sunlight. 1t is
easy to intercept beam of light.
Of course. Yet it would be risky. To
do 1t would mean getting in between
in the direct line Of light. 1t would
mean instant discovery, and trouble
to follow. That is the only way you
can tap the sunphone. No one can
overhear conversations over it. 1t is
quite safe and quite secret-safer and
more secret than wireless. though
not. Of course, comparable to it in
range. The sunphone is superior to
the .heliograph, as 1t needs no code.
The idea of using light as a vehicle
for speech Is not new. The way by
which the problem has been solved
is new. That is all, but 1t is every
thing. 1t all hangs around selenium.
Selenium is an element of the sulphur
group. Its chief source is the flue
dust obtained in burning pyrites in
the making Of sulphuric acid by the
1eaden chamber process. 1t is q gray,
crystalline, semi-metallic substance.
It's chief property is its response to
light. 1t conducts electricity. as 1
have already explained. in proportion
to the brightness Of the light which is
brought to bear upon it. This char
acteristic has long been we'll known.
For the last thirty or forty years
various attempts have been made to
use 1t for the purpose to which DR..
Rankine has now managed to apply it.
It is an interesting fact that Selene
was the moon goddess of Greek my
thology. She was sister to Phoebus, the
sun god. and rode across the heavens
in a chariot drawn by two white
horses. In view of DR.. RANKINGS dis
covery who will say that selenium Is
not we'll named?
Two main methods of solving this
problem Of speech transmission by
light have been tried by previous ex-
perimenters. One was to cause fluc-
tuations varying with the sounds of
speech in the source of the light it.
self. The other was to produce the
fluctuations indirectly by causing the
speech to interrupt the light after 1t
had left it's source. The second is the
one used by Dr. Rankine. BY using
this he Is able TO employ sunlight as
well as artificial light. If he had kept
to the other method sunlight obvi-
ously could not have been used. The
average frequency OF speech sounds
Is about 500 per second. That means
that tho brightness OF tho beam of
light must alternate between maxi
mum and minimum every one-five-
hundredths Of a second.
\ y u X
TN a patent specification of 1888
Graham Bell describes two devices
- writes DIR. Rankine in
his paper. "In the first the proposal
10 to allow the beam Of light to pass
in succession through two grids con-
Fsisting Of equal parallel astrffJs alter
JECtlg! A1g s5s - ytI%x tAflt 1M