House Doctor: Jeff Howell

IS IT TRUE, as some people say, that light bulbs blow more easily in cold weather? The only logical reason for this would be that the greater temperature difference between the "on" and "off" states of the light causes more stress on the filament - the thin wire inside the bulb that glows white hot and gives off the light. But, since the tungsten filament of an incandescent bulb glows at around 2,500C, the fact that it starts off at 10C rather than 20C, say, would appear to be neither here nor there. We must face the fact that light bulbs break more often in the winter months because there are more hours of darkness to eat up their 1,000- hour average life spans.

A more significant factor in bulb life is which way up it goes in the fitting. Bulbs are designed to hang downwards, and if they point upwards or sideways they don't last as long. In all bulbs the filament becomes oxidised and brittle with use, and when the bulb is pointing upwards or sideways the filament can eventually collapse under its own weight. The filaments in downwards-hanging bulbs will usually only break before their time if you bash them.

Another disadvantage of upward-facing bulbs is that when they do blow, they tend to short out the whole circuit, blowing the fuse or MCB ("miniature circuit breaker" or "trip switch") on the fuse board, and plunging the whole place into darkness. This is because bits of the broken filament fall across the two bulb terminals and cause a surge in power; if the bulb is hanging downwards then the broken bits land harmlessly in the glass.

Lately, however, even downward-hanging bulbs have started to trip circuits, because of a lowering in manufacturing standards. Light bulbs used to have a little built-in fuse - a very thin strip of wire in the bayonet cap - which would burn out first, leaving the rest of the filament intact. But pressure on costs from overseas manufacturers has meant that most modern bulbs are unfused, and can blow at any point along the filament. You can buy light bulbs for 20p, but they won't last as long and they'll blow your fuses. You never, ever, get something for nothing.

The basic lighting in most rooms consists of a single pendant fitting hanging from the ceiling. When electricity was first introduced, in Edwardian times, it was feared that the new-fangled bright lights would cast revealing silhouettes of the occupants onto the curtains, so in the bedrooms and bathrooms of older homes the light fitting is usually close to the window.

This practice is not followed by modern housebuilders. The reason is nothing to do with changing concepts of privacy, however. Current methods require three separate cables to each ceiling light fitting: by siting the pendant fitting in the centre of the bedroom ceiling rather than 2m closer to the window, the electrical contractor can save a tenner on a new three-bedroom house - not to be sneezed at with the tight margins of the modern building industry.

'Struck Off - The First Year of Doctor on the House', by Jeff Howell is available from Nosecone Publications, P O Box 24650, London E9 7XQ, price pounds 9 inc postage.

Jeff@doctoronthehouse.demon.co.uk