Evening star., 1943-03-14

Heavy rain has chased late straggle
setup for the prowler whose vague
He has just watched the guard pat
again for another half hour. He
jimmying the window.
But half block away in the pl
ing the minute the skulker crosses
the yard. A red light glows on a
panel and buzzer sounds a muted
note. Glancing at the number un-
der the red light, four husky guards
grab riot guns and race for the very
spot where the skulker is just break
ing the window lock. Before he
realizes what is happening he's
handcuffed and on his way to jail
Enemy Agent.
Maybe he Is just a thief, but more
likely he's an enemy agent intent
on destroying the valuable supplies
in the warehouse. He has been
caught by the black magic of an
entirely new. method of burglar de-
lection. He broke through fence
of black light tuned like a radio, so
that nothing he could have done.
even If he knew exactly where 1t
was, would have stopped the alarm.
The men who invented the new
system have tried in vain to And 4
method of "fooling" the invisible
fence. Seventy-five crooks have
tried to break into plants equipped
with tuned black light fences; 55 of
them are doing time in Jail. The
others were scared away.
The principle of this infallible
protection Is not entirely new. 1t is
the electric eye or photoelectric cell
which you have seen used to open
and close store doors without any
one touching them. A beam of in
visible intra-red rays is generated
by passing the light from an electric
bulb through a filter which removes
all Of the light we can see. This in
visible or BLACK light" is focused
through lens upon a tube that
locks somewhat like radio tube.
AS long as the ''black light" shines
on the tube, it allows a current to
pass through 1t from a battery and
thus hold magnetic switch or relay
open. Cut the black light" beam
by walking through it and the
switch clicks shut and turns on the
alarm light and buzzer.
The new feature that makes the
alarm beam tamper proof is that it's
tuned" A slotted disk revolves in
front Of the source of black light" :"
interrupting 1t at any desired fire.
quency. The receiving bulb is tuned
SO that it will respond only to cne
particular rate of interruption. Be-
fore this tuning system was invented
smart burglar could have his helper
shine the beam from a special fash
light into the receiving "eye." Then
he'd calmly walk through the beam
and nothing would happen because
the ffashlight had fooled the "eye."
Wave Of Bullets.
The saboteur we mentioned above
cut cne OF the many beams of the
alarm system gridironing the plant
grounds. The guards knew eactly
where to look for him because each
beam has its own signal number on
the panel. In some plants, the in
slant he stepped through the beam,
f1oodlights would have blazed on, a
siren would have commenced to howl
and every gate into the plant would
have automatically slammed shut.
But the silent signal is gaining favor
because 1t always causes the capture
of the prowler.
Tuned black light fences as long
as 1000 feet are in use. BY using
mirrors the fence ray can be made to
turn corners. Mist or rain has no
effect on these mirrors because each
Is kept dry by an electric heater. If
prowier knows of the existence of
the ray and tries to beat 1t by break
ing a mirror, he only succeeds in
turning in an alarm.
Wartime uses of tuned black light
make rivers and harbors safe from
sneak raids by enemy ships. The
instant the enemy ship cuts the in
visible beam, mines concealed under
water are set of and sink the In
vader. The same system can be used
to prevent surprise attacks against
land Notifications. A grid of beams
can be set in the path Of enemy
patrols which, when broken by Creep
ing men, will put automatic cannon
and machine guns into action and
sweep the patrol with a destroying
wave CT bullets. Established along
the beats OF sentries guarding bridges
or reservoirs. the invisible beam is
OF great value in stopping raiders be-
cause. unlike the sentry, the beam
can see in the dark.