The Dangerous Book for Boys. Conn Iggulden

until you arrived the day before. Obviously this is not possible, and so crossing the line going west would add a day to the date. Complex? Well, yes, a little, but this is the world and the systems we made to control it.

      Like latitude, longitude is broken down into a three-figure location of degrees, minutes and seconds. Common practice puts the latitude figures first, but it’s always given away by the North or South letter, so they can’t really be confused. A full six-figure location will look something like these:

38° 53’ 23” N, 77° 00’ 27”	W Washington DC
39° 17’ 00” N, 22° 23’ 00” E	Pharsalus, Greece, where Julius Caesar beat Pompey and ended the civil war.
39° 57’ 00” N, 26° 15’ 00” E	Troy

5. HOW DO YOU TELL THE AGE OF A TREE?

      You cut it down and count the rings. For each year of growth, a dark and a light ring of new wood is created. The two bands together are known as the ‘annual ring’. The lighter part is formed in spring and early summer when the wood cells are bigger and have thinner walls which look lighter. In autumn and winter, trees produce smaller cells with thicker walls which look darker. They vary in width depending on growing conditions, so a tree stump can be a climate record for the life of the tree – sometimes even centuries. The age of a tree, therefore, can be told by counting the annual rings.

       Making a Battery

      A BATTERY AT ITS SIMPLEST is a cathode (the positive end), an anode (the negative end) and electrolyte (the bit in the middle). There are quite a few different combinations out there. Electricity is the movement of electrons, tiny negatively charged particles. The anode tends to be made of a substance that gives up electrons easily – like zinc, which gives up two electrons per zinc atom. The cathode tends to be made of substances that accept electrons easily, like copper.

      The electrolyte inside can be a liquid, a gel or a paste. All that matters is that it contains positive and negatively charged ions that flow when the anode and cathode are activated. When the Italian physicist Alessandro Volta made the first battery, he used copper for the cathode, zinc for the anode and an electrolyte of blotting paper and sea water. His name gives us the word ‘volt’, as in a 12-volt car battery. If you think of electricity as a water pipe, a volt will be the speed of the water, but it also needs a big hole to flow through – or ‘amps’. You can have enough voltage to make your hair stand on end, but without amps, it won’t do more than cause a tiny spark. A house supply, however, has 240 volts and enough amps to kill you as dead as a doornail.

      You will need

       Ten two-pence pieces.

       Metal kitchen foil.

       Blotting paper.

       Two pieces of copper wire (taken from any electrical wire or flex).

       Malt vinegar.

       Salt.

       Bowl.

       LED – a light emitting diode (available from model and hardware shops).

       Masking tape.

      The copper coin will be the cathode, the foil the anode.

      Cut the foil and blotting paper into circles so they can be stacked on top of each other. The blotting paper will be soaked in the vinegar, but it is also there to prevent the metals touching – so cut those paper circles a little larger than the foil or coins.

      1. Mix vinegar and a little salt together in the bowl. Vinegar is acetic acid and all acids can be used as an electrolyte. Sulphuric acid is found in car batteries, but don’t fool around with something that powerful. It eats clothing and can burn skin – unlike vinegar, which goes on your chips.

      Common salt is sodium chloride, a combination of a positive and negative ion (Na+ and Cl–). These will separate in the electrolyte, increasing its strength.

      2. Soak your circles of blotting paper in the ion-rich electrolyte.

      3. With the masking tape, attach the end of one wire to the underneath of a foil disc. This is the negative terminal. Now stack in this sequence – foil, paper, coin, foil, paper, coin. Each combination is its own tiny battery – but to light even an LED (light-emitting diode) you’ll need quite a few. A car battery tends to have six of these, but with a much larger surface area for each ‘cell’. As a general rule, the bigger a battery is, the more power it has. (Power measured in Watts = amps × volts.)

      All the positive ions will go to one terminal, all the negative ions to the other. In effect, you are charging your battery.

      4. When you have a stack, you can attach a wire to the last coin with tape. This will be the positive terminal. They can now light an LED, as in the picture below, or with enough coin batteries, even a small bulb.

      There may come a time when batteries go on to a new generation, but if you can understand the battery you have just made, you can understand every type of battery currently available, from nickel-cadmium, to lithium-ion, from rechargeable phone batteries, to the ones that drive toy rabbits. You won’t hear acid sloshing in alkaline batteries, where a paste or gel is used, but the principles are identical.

       Conkers

      THE FIRST THING TO DO is spend an autumn afternoon throwing sticks into the branches of horse chestnut trees. For conkers, the bigger the better is a good rule and any that still have white spots should be given to friends – they are useless. Collect more than you need. This is only your practice year. Next year, you’ll take out all the ones you prepared and win constantly, but this is the year you learn your skills.

      The trickiest part is making a neat, small hole. You will be tempted to use a nail, a spike or anything else with a point. This does work, but there’s always a chance of spoiling a good conker in the process. Better to get your dad to use a drill on them. Don’t try that one yourself. The conkers spin round at high speed, or crack when you put them in the vice. Much better to ask an adult to do it, but give them your worst conkers to start with until they have learned the knack.

      Once you have your holes, you need a strong trainer lace. Don’t waste time with the ones from school shoes – they just cut into the conker.

      Getting the lace through is always tricky and takes patience. Start by licking the end and twisting it into the hole, pushing and twisting to feed it in. Once you have it started, you’ll need a bit of stiff wire to poke it through. Don’t be tempted to use a fork, they never worked for us. Prod and twist until you can see the other end and then tweak it through with the wire. It’s a good moment when you can finally pull the whole length through.