Science: Bones and muscles, ploughs and phones

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Q What are bones like inside?

Large bones, such as the thigh bone, have a hard, thick, outer layer and a soft middle called the marrow. The cylinder formed by large bones gives them great strength: they can support heavy weights without bending and breaking. Bone needs to be strong to stand up to the stresses of supporting and moving the body. Yet the bone itself is not solid. It forms a microscopic honeycomb which leaves lots of spaces for nerves and blood vessels.

Under a microscope, it is clear there are two basic sorts of bone. Compact bone is dense and heavy, but very strong. It is found in load-bearing areas such as the shafts of the long bones of the body (eg the main part of the femur, or thigh bone).

The second type of bone is called spongy bone. This has a much more open structure and tends to be lighter than compact bone. Spongy bone is found in growing regions and in large masses of bone such as the head of the femur, which fits into the hip socket.

Q How much of my body is muscle?

In a normal adult, about two-fifths of the body weight is muscle. The actual figure depends on the individual. Most teenagers have slightly less muscle: more forms as your body matures.

Q How do metal detectors work?

To sniff out metallic objects, they all rely on a metallic "search" coil (sometimes called a search loop) with an alternating current running through it. The AC causes a uniform alternating electromagnetic field around the coil. But when that field passes over a metallic object, it induces tiny currents - called eddy currents - in the object. That weakens and distorts the detector's field (because the eddy current comes from the field's energy). When the field passes over a magnetic material (say, iron or magnetite) it strengthens the field. An audible note linked to the energy in the coil then alerts you when the search loop passes over a conducting metallic object (less energy) or a magnetic material (more energy). Thus you can also distinguish between metallic and magnetic objects.

Q Who invented the plough?

Joseph Foljambe took out the first patent on an iron-sheathed plough in 1720, and was credited with its discovery even though such ploughs had been used in England since before Roman times. However, the Chinese had produced iron ploughshares more than two millennia before, in the 6th century BC. By the third century BC they had developed the kuan, made of malleable cast iron, which was much more sturdy.

The earliest metal (bronze) ploughs, dating from the 16th century BC, come from Tonkin in Vietnam. The third millennium BC shows us pictorial representations of ploughs from Uruk (present-day Iran) and Chinese triangular stone ploughshares date back as far as the early fifth millennium BC. It is ironic that the agricultural revolution that led to the industrial revolution and the supremacy of the west was based on a Chinese invention.

Q When was the first mobile phone sold?

In 1979 the first commercial network of cellular telephones was set up in Tokyo. Later in the same year, AT&T's Bell Laboratories tested a cellular system on 2,000 users in Chicago. By 1983 the first regular US cellular telephone system was installed. Those systems were all analogue: the first digital phones were not on offer until 1992.

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