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Off your trolley

Last week my shopping trolley ground to a halt when I tried to push it beyond the confines of the store car park. The security guard told me that the trolley was protected against theft and that once beyond a certain point its wheels ceased to function. How does such a system work?

• In Britain there are three systems for halting shopping trolleys. In the first, radio signals emitted from a cable installed around the supermarket grounds triggers a shutter that comes down between the wheel and the ground when the trolley reaches the car park limits. The second system is also based on a radio signal, but this version triggers a brake that locks the wheel. Miscreants can still drag the trolley but obviously it causes a flat spot on the tyre if they try. Both these systems were created in California.

In the third system each trolley is fitted with two devices that respond to a specific magnetic field when they travel beyond a certain point. This system releases a brake that drops onto the ground and makes the trolley much harder to push or drag than either of the first two systems. I am delighted to say that I invented this magnetic system, called the Radlok, which now sells well from Calgary to Auckland, and also in your correspondent’s home city of Edinburgh.

Andrew GrayGray

Matter (Alpha) Limited

On one of the front wheels and one of the back wheels of the trolley there is a small plastic casing. Inside this casing is a rubberised foot connected to a hinge. Across all entrances and exits of the perimeter of the store car park there is a magnetised steel strip or bricks impregnated with magnets.

Inside the casing on the trolley wheel there is a magnetically operated trip switch. As the mechanism passes over the magnetised metal or bricks, the rubberised foot drops. This makes the trolley practically immovable. When supermarket staff retrieve the trolley, they simply use a small plastic key to lift the foot back into the mechanism and lock it into place.

As somebody who works in a supermarket, I can appreciate that this is a rather good method of theft prevention.

Murray Thomson

Dorset

The system Murray Thomson describes appears to be of the third kind, a magnetic-type trolley brake – Ed

Red sky at night

Earlier this year I was watching a night-time thunderstorm. The sky was lit up by some of the most sustained flashes I have ever seen, some lasting up to three seconds. On the longest of these sustained flashes there was a strange phenomenon. The white light grew more and more intense, then suddenly changed to red, just before it faded. What could have caused this?

• Lightning is an electrical discharge, or plasma, generated by a flow of electrical current through the atmosphere. In general, a plasma is created when a gas heats up enough to become ionised – in other words, a significant fraction of the atoms and molecules lose their electrons. As a consequence, the gas conducts electricity and emits electromagnetic radiation: the two principal observable features of a plasma.

A lightning plasma is quite dense, because the degree of ionisation in the air near the lightning is high. It therefore gives off a lot of infrared, visible light and ultraviolet. A lightning flash consists mainly of light emitted at all wavelengths, plus line and band emissions at characteristic wavelengths from the atoms and molecules present in the surrounding air – mostly argon, oxygen, nitrogen and water. The lightning we see contains contributions from all these sources and the colour depends on the kind of atoms in the air.

A discharge in dry air looks white because there are few strong visible lines – those which exist are mainly blue and violet – and the bulk of the visible light is emitted in a continuous spectrum. If water vapour is present, then the hydrogen atoms in the water create a very strong red line (known as the Balmer alpha line) that can dominate the visible line emission. If this line is superimposed on the white background created by the other atoms in the plasma, it would explain the reddish appearance of some lightning flashes. The reddish colour will be easier to see at night. The variability in the amount of red in a series of lightning flashes is explained by variations in the amount of water in the air. In addition, water vapour or tiny water droplets are more easily ionised and their hydrogen atoms more easily excited than the hydrogen in large water droplets – the latter must first evaporate, which takes longer than the duration of a single flash. It is possible that large droplets were broken up during the long series of localised flashes reported by your correspondent, thereby creating an exceptional redness.

The colour of lightning can easily be simulated in the lab using a focused laser beam to create a breakdown spark in air. If the air is dry, the spark is purplish white. If the air contains water vapour, then the spark has a reddish tint, which becomes stronger at higher relative humidity.

Donald McNeill

Pittsburgh, Pennsylvania

This week’ question

Floundering about

How do certain animals, such as the flounder, change their colour to match their background (see Picture below)? More specifically, if you made a tiny blindfold for the flounder, would it still be able to match its surroundings?

Nick Axworthy, by email, no address supplied