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A PHYSICIST WRITES . . .

(January 2008)

Itís December as I start writing this column ó probably the month when you are most aware of the shortness of daylight, before the winter fog, frost and occasional snow really arrive and add to all the other hazards of darkness on the road. If youíre a driver, there is a simple test prescribed for your vision: you must be able to read a vehicle number-plate from a distance of 20 metres, in good daylight. I could write pages on how inadequate this test is as a measure of your ability to see clearly in all directions in any light. But for now Iím particularly concerned about how my eyes perform as it gets dark.

If the brightness of the scene in front of you decreases, the pupils of your eyes open up to compensate and let more light in. But this reduces the clarity of the image on the retina at the back of the eye (exactly as in a camera when the aperture of the lens is widened too much).

As it gets darker still, the eyes increase their sensitivity by making use of a different set of light-detecting cells in the retina. But these cells canít easily distinguish between colours, and they are mostly off-centre ó so you see even less clearly in the exact direction you are looking.

Then there is the problem of glare from bright lights approaching. And lastly, everything looks different in the dark anyway! The kerb might show up only as a faint reflection of the light from someone elseís headlights, the road surface is often hard to see, cyclists and pedestrians might only be indicated by the hint of a silhouette, and your reactions to whatever you do see could be slow too (simply because it is indistinct). Really itís a miracle that we donít have constant accidents when driving at night.

What bothers me is that as far as I know thereís no simple standard way of testing how good peopleís eyes are in night conditions. I can understand that it would not be easy to do ó for one thing, you would have to sit in darkness first until your eyes had fully adapted to it. But I would like to know if I can see better or worse in the dark than the average person. At present, Iíve no idea.

Changing the subject now, is it actually safe to do a jump-start either to your vehicle if the batteryís flat, or to someone elseís from yours? The answer has to be: read the manual! Modern cars have computer control and other electronics that are in danger of being destroyed by voltage spikes when the jump-leads are connected, or when the other car is started.

But letís suppose you are willing to take the risk: should the car with the good battery be running or not, when you connect/disconnect the leads? Should it be started at all, even, while you are trying to get the other car going? Well, some advice Iíve read says yes, some says no. I couldnít possibly comment. You must decide.

The question I can deal with ó though I confess I had the wrong answer in my head for years ó is which jump-lead should be attached first: the negative (black) or the positive (red)? All electricians, electronics experts and physicists know the importance of making reliable earthing connections to equipment and circuits, for safety and other reasons. Shouldnít you connect up the black jump-lead first, therefore, joining the two negative battery terminals and thus linking one car chassis to the other in a nice safe structure?

No!! If you did this, and then proceeded to connect the red lead to one of the positive terminals, the other end of this lead would also be at +12 volts, and if you accidentally touched it on any part of either car (other than the fourth battery terminal) the resulting bang would probably bring the Bomb Squad at the double. So you must join the two positive terminals first, with the red lead.

Next, fasten the black lead to the negative terminal on the good battery. The other end should now be safely clamped not to the same terminal on the dead battery, but to a suitable corner of the engine, well away from the battery and from the fuel system. This is because a (small) spark is likely when you apply the clamp. Afterwards, disconnect the leads in the exact reverse order...

...Breaking off from typing, I discovered on the internet that thereís a whole range of portable starter-packs available, with internal battery and jump leads. The bargain of the month was one in Maplinís, featuring also a tyre pump, a lamp, and a 12 V cigar-lighter output socket: half-price at £19.99. Irresistible! So I visited the Reading store: sorry sir, itís out of stock and no chance of more coming in. Back home I found the Maplin website, punched in an order, and the pack was delivered three days later (on a Sunday afternoon), all for £22.97.

OK, it weighs about five kilograms and the jump-leads are short, but in most flat-battery situations it must be easier to restart this way than from another car (though your electronics might still be at risk ó see above). The internal pump works about as fast as a foot-pump when youíre fit. If you bought an invertor too, to run off the 12 V socket, you could power almost anything anywhere at 240 V. You can recharge the starter-pack (slowly) from the mains or from a running car (the cigar socket).

But alas: itís January now as I finish writing this column, and on the website the starterís back to £39.99 (inclusive).

Peter Soul

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