Stop thief
Question: I have noticed that electrical goods stores use a form of adhesive
metal coil as a security tag. How do these tags set off the door alarms and how
are they deactivated before a legitimate customer leaves the shop?
Answer: The coils are the antennae of a very simple, disposable radio circuit
that is based on a tiny diode or tuned resistor-capacitor circuit. The gates at
the shop door emit a radio signal that the circuit is tuned to, so the circuit
resonates and re-radiates a modified signal. When the gates pick this up they
trigger the alarm. Blasting the coil with a strong radio signal destroys the
circuit and deactivates the tag.
Other systems involve strips of 鈥渟oft鈥� and 鈥渉ard鈥� magnetic materials. A soft
material (like steel) stays magnetised only up to a certain limit. If coils in
the shop gates temporarily magnetise it beyond this limit, a small change in the
field from the coils can be detected, which triggers the alarm. To deactivate
the soft strip, a coil in the counter permanently magnetises a neighbouring hard
strip in the tag. The field it creates keeps the soft material at its limit, so
there鈥檚 no change in the field from the gate when it passes through.
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Similar to this device is one where an oscillating magnetic field created by
the gates forces a so-called magnetostrictive strip to ring mechanically, like a
bell. This sympathetic ringing creates a distinctive radio signal.
A neighbouring magnetised strip ensures the device resonates at this precise
frequency. If this strip is de-magnetised at the counter, the magnetostrictive
strip radiates the wrong signal and the goods can pass through the gates.
Stuart Nevill
Brighton, East Sussex
Answer: Anti-shoplifting tags are based on the harmonic radar principle,
which was first used by German rocket scientists at Peenem眉nde in the
Second World War to track their V2 rockets by radar.
The tag consists of a coiled printed antenna with a diode fixed in-between
the two arms of the antenna. The diode doubles the frequency of the incident
electromagnetic field.
At the doorway of the shop is a transmitter that radiates a specific
frequency and a detector that picks up the new frequency or 鈥渟econd harmonic鈥�
when a tag is moved through the doorway, triggering the alarm. The tags are
deactivated by a high-current pulse at the counter, which destroys the
diode.
The trouble with the harmonic radar principle is that its range is rather
short. Nevertheless, the method was developed for other applications. The US
military developed metal re-radiation. With this system, a third harmonic is
detected in the presence of any metal-metal or oxide-metal junction. Any
corroding metallic object, such as a rusty nail, has such elements. The military
hoped this could be used to detect camouflaged tanks.
In Sweden, they have used the principle to detect avalanche victims. Skiers
are given a special tag that can be pinpointed if they are buried under snow.
Remarkably, the same technique has also been used to trace the movement of
ground beetles in the dark. Flying bees and butterflies have been tracked in a
similar way.
Another application is the 鈥渘on-linear junction detector鈥�, which is used to
search for hidden electronic equipment. In this application the re-radiated
harmonics originate from non-linear junctions such as diodes and transistors in
electronic spying equipment.
The harmonic radar principle has also been used to develop a collision
avoidance system. To make this work properly, a harmonic re-radiation antenna
tag would have to be fitted to the rear of every car.
Thomas Wriedt
Institute for Materials Engineering
Bremen, Germany
Sex thought
Question: I have heard many times that 鈥渕en think about sex every 8 seconds鈥�.
I am interested to know whether this is based on experimental observation and,
if so, how would you carry out such an experiment?
All our attempts to track down any original research on this topic suggest
that it is an urban myth. The story appears to have been around since at least
1997. It has featured in a beer advertisement, been the subject of a complaint
when aired on television at an inappropriate time, and discussed on innumerable
Internet forums. But we can鈥檛 find where it all started.
It鈥檚 also extremely unlikely to be true in a literal sense. That鈥檚 because
most thoughts about sex probably last longer than 8 seconds, although if we were
to twist that a little and suggest that on one in eight random occasions a man
is thinking about sex, then it might be true. Then, of course, we would have to
ask if that is a lifetime average, or for a particular age group, and whether
this applies to a solitary male in an isolated room or out in the world with
lots of females around.
Anyone who really wants to know could follow the advice of the engineer
below, or simply try asking a large random group of males what they are thinking
about, under various controlled conditions鈥擡d
Answer: A technique called correlated signature analysis, which is commonly
used in engineering research, could be applied here.
We can measure the time-dependence of a number of parameters that are known
to be associated with arousal, such as skin resistance (you can confuse a lie
detector by thinking about sex), penile blood flow, and brain or pelvic neural
electrical activity.
By imposing suitable stimuli鈥攕ay a series of erotic images randomly
interspersed in an otherwise neutral slide show鈥攚e can induce 鈥渇orced鈥�
arousal and study the time-correlated response of our chosen signals: this gives
us the electrophysiological signature associated with thinking about sex.
We then look for the same signature in the absence of an imposed stimulus.
There is plenty of suitable multichannel recording equipment and software on the
market.
Unfortunately, most of us are subject to other stimuli during waking hours.
But these occur at random times, are usually correlated with specific external
events, and are unlikely to produce the same multi-parameter signature, so it
may be feasible to discriminate a regularly recurrent 鈥渟ex thought鈥� signal from
the noise of everyday life.
Alan Calverd
Bishop鈥檚 Stortford, Hertfordshire
This week鈥檚 question
Family matter: My observations suggest that in Britain there are clusters of
mole colonies with large mole-free regions in-between. How did distant colonies
get started? Is each colony hopelessly inbred? Are moles a British phenomenon?
Or are there similar continental moles and, if so, how did their communities
spread?
Philip Welsby
Edinburgh