Lighting up
When you go to the toilet on an airliner, bolting the door operates the light inside the cubicle. However, after you complete the circuit by sliding the bolt, the light takes a couple of seconds to come on. Why does it do this?
鈥� It takes a couple seconds for the low-power, low-voltage fluorescent light fixture to supply enough energy to the tube to establish an arc across (or through) the gas used in the light fixture. This is because the small quantity of mercury that is used to create the arc needs to heat up just enough to start ionising.
The mercury arc, which produces light at a wavelength of 337.1 nanometres 鈥� just outside the visible spectrum in the ultraviolet range 鈥� excites the atoms in the coating inside the tube and they then emit photons in the visible range. The delay happens as the arc is being established.
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Unlike the electricity supply that powers modern fluorescent light fixtures in the home and office, the 24-volt electrical systems in aircraft are less 鈥渟tiff鈥�, meaning they generate less energy. But the surge needed to light an instant-start fluorescent tube would create a significant voltage dip, lasting a few tens of milliseconds, that could play havoc with other aircraft devices. This is the reason the light has a 鈥渟oft start鈥�.
Dan Schwartz
Maple Shade, New Jersey, US
It鈥檚 a knockout
Why do you lose consciousness when knocked on the head?
鈥� A knockout blow generates swirling motions in the brain, causing shear that breaks or strains nerve fibres. Brain tissue may recover slowly, if at all, or die. However, the brain compensates for the destruction of neurons in this way by retraining alternative pathways to take over lost functions as far as possible. This means that isolated incidents may leave no obvious damage, but repeated harm accumulates in what is colloquially known as punch-drunkenness. Not all such damage results in impaired consciousness, however. It may instead manifest itself as the gradual discovery that functions or memories have been lost.
The longer the neural pathways the more vulnerable they are to the damage caused by shear and strain, so neurons that run between different parts of the brain are particularly vulnerable, and these include the connections between the sensory inputs and the control parts of the brain.
In particular, the reticular formation runs along and between sensory pathways and its fibres have a lot of interconnections. It is a network that monitors inputs and controls mental arousal and is crucial to consciousness, so a blow to this area can interfere primarily with this mental state. Such damage causes confusion or loss of consciousness until the system has recovered (if recovery is possible) or until alternative pathways have taken over.
Jon Richfield
Somerset West, South Africa
鈥� Boxers sometimes lose consciousness when a seemingly harmless blow connects with the chin. When the jaw is even slightly open, the force of the punch is transmitted to the base of the skull across a small area of contact from the jawbone. The resultant large stress jolts the brainstem (which is associated with consciousness) where it passes through an opening in the base of the skull, and the boxer will lose consciousness.
When the teeth are clenched, this contact between the jawbone and base of the skull is lost, and the force passes between the apposed teeth and into the rest of the skull bones. This weakens the stresses the brainstem sustains, and loss of consciousness is less likely.
Chris Jones
Department of Anatomical Sciences
University of Adelaide, Australia
This week鈥檚 questions
Woodland wonder
The ice crystals in the photo (below) were found on small branches lying on the ground in mixed woodland. The crystals had only formed where the bark was missing and there was very little frost elsewhere on the ground. Can anybody explain how they formed?
David Meadows,
Yeovil, Somerset, UK
Bendy bond
My 12-year-old son has been asking me how flexible fridge magnets are made (such as the ones with printed advertising on one side). And what sort of material are these magnets made from?
Colin and Daniel Disseldorp
Somerville, Victoria,
Australia