Guglielmo Marconi, of Bologna, Italy, was the first to prove that radio signals could be sent over long distances. Radio is the radiation and detection of signals spread through space as electromagnetic waves to convey information.
It was first called wireless telegraphy because it duplicated the effect of telegraphy without using wires. On December 21, 1901, Marconi successfully sent Morse code signals from Newfoundland to England.
A fruit is the part of the plant that nourishes and protects new seeds as they grow. The plant’s ovaries develop into fruit once the eggs inside have been fertilized by pollen. Some plants produce juicy fruit, such as peaches, pears, apples, lemons, and oranges.
Others produce dry fruit, such as nuts and pea pods. If an animal doesn’t eat the fruit, or a human doesn’t pick it off, it falls to the ground and decays and fertilizes the soil where a new seed will grow.
A bicycle is a simple device that increases the power that you have in the muscles of your legs, taking you faster and farther than you could ever run. When you push the pedals of your bike around once, the pedal sprocket—the wheel with teeth to which the pedals are attached—goes around once, too. But it pulls a chain along, one that is connected to a much smaller sprocket (with fewer teeth to grip each link of the chain) in the center of your bike’s rear wheel. This smaller sprocket moves around a number of times for each single turn of your pedals, moving your bike wheels a lot faster than you’re moving your feet! Some bicycles have several “speeds,” which means that they have a number of gears (called derailleurs) that vary the rate at which the wheels turn. These extra sprockets are located at the pedals and rear wheel of a bike, where levers move the driving chain sideways, from one to another.
A special spring system keeps the chain tight when it changes from a larger to a smaller sprocket. Although you might think that a rider would always want the wheels of his or her bike to move as fast as possible for each pedal turn, that is not always the case. When going uphill, for instance, a rider can get more force out of a wheel that turns fewer times, making the task easier.
Since ancient times, soldiers have worn special clothing or armor to protect themselves during warfare. Hard materials like leather, wood, shells, and even woven reeds were used to give soldiers extra protection against enemy arrows. Metal started to be used for armor about 3,500 years ago, by warriors in the Middle East. By the time of the ancient Greeks, about 1,000 years later, soldiers were well protected, wearing large pieces of metal on their chests and backs, shin guards, and metal helmets, and they carried metal shields. Soon armored clothing, garments with metal strips and plates attached, began to be made for soldiers. Then chain mail, a type of metal cloth, was developed. Made of small metal rings linked together, chain mail was much more flexible than metal plates, but could not withstand the force of larger weapons, like lances. So full suits of armor made of steel plates, hinged at the knees and the elbows, came into use around the fourteenth century. Soldiers were covered with steel from head to toe, with heavy metal helmets covering their faces, heads, and necks. A warrior could see and breathe through small slits or openings in the helmet’s visor, a movable metal flap that could be lifted up.
(Only important or wealthy warriors could afford this kind of elaborate armor.) Suits of armor weighed so much that the soldiers or knights who wore them usually could not move around in them very well; they wore such armor mostly when they fought on horseback. Even the horses sometimes wore armor. As the methods and weapons of warfare changed, clumsy personal armor was no longer useful. It became far more important for soldiers to be able to move quickly and easily. Today’s soldiers usually wear cloth uniforms, body armor, and steel helmets. But armor is used on war vehicles like tanks, naval vessels, and aircraft. The bulletproof vests that police officers use are also a type of armor.
Disease and poverty exist all over the world. The areas with the most disease and poverty are countries in sub-Saharan Africa and the Asia-Pacific region. In these areas, people do not have enough food to eat, water to drink, or money to live.
They have diseases like AIDS, which weakens the immune system, and cholera, an intestinal infection. In the United States, about 33 million people live in poverty, according to government statistics. Almost 12 million of these people are children, and about 3.5 million were age 65 or older.
America’s founders did debate a bit as to whether or not to force children to attend schools, and they decided to leave such decisions to individual families and local and state governments. The words “education” and “school” do not appear in any of our founding documents, such as the Declaration of Independence, the Constitution, or the Bill of Rights.
Some of our most famous inventors, writers, and politicians were self-taught, learning through mentoring or apprenticeships, conversation, and reading. In 1850, Massachusetts became the first state to institute a compulsory schooling law.
The word shift means to change or rearrange, which is why we call those who work during differing blocks of time “shift workers.” This use of the word shift also applies to an individual’s ability to change or adapt.
Therefore, if you’re “shiftless” you lack the initiative or resources to change with the circumstances. On the other hand, someone who is “shifty” is too adept at change and isn’t to be trusted.
Television works through a series of complicated processes. It starts with a television camera, which takes pictures of scenes. Photo cells inside the camera change the pictures to electrical signals. At the same time, a microphone records sounds that are occurring during the scenes. A vibrating magnet in the microphone changes these sounds into electrical signals, too. Some television shows, like news reports, are recorded live, which means that they are broadcast to homes as they occur. But most of the television programs that we watch are recorded, which means that they are put on videotape and sent out later. The electrical signals of sound and pictures are stored as magnetic signals on videotape, which are converted back to electrical signals when played. Before a program is broadcast, its electrical picture and sound signals are run through a device called a television transmitter. With the help of strong magnets, the transformer turns the electrical signals into invisible bands of energy called radio waves (similar to visible light waves), which can travel great distances through the air. They can travel directly to outdoor television antennae, which catch the waves and send them to television sets that change them into pictures and sounds again.
Cable companies send electrical picture and sound signals through cables directly to homes. When broadcasting to distant places, communication satellites that orbit Earth are used to bounce or return the waves back to Earth, extending their travel distance. Satellites are necessary because radio waves move in straight lines and cannot bend around the world. When an antenna or satellite dish receives radio waves, it changes them back into electrical signals. A speaker in a television set changes some of the signals back into sound. The pictures are reproduced by special guns at the back of a television set that shoot electron beams at the screen, causing it to glow with tiny dots of different colors. Viewed together, the dots look like a regular picture. The individual pictures that make up a scene are broadcast and received, one after another, at a pace so quick that it looks like continuous action is occurring on the screen. The entire process happens very fast because television stations and broadcast towers are all around and because radio waves travel very quickly, at the speed of light. Radio programs broadcast talk and music across the airwaves using the same technology.
Technically, no. While it is responsible for receiving and transmitting all messages of pain for the whole body, the brain itself does not have pain receptors.
That means that, if you could somehow gain access to another person’s brain, you could poke it or pinch it and that person would not feel the pain.
Glow-in-the-dark stickers, stars, toys, and clothes, all work by absorbing light and emitting it later. These items contain phosphors, substances such as zinc sulfide that radiate visible light after being energized by natural light. Phosphorescent materials continue to glow after the energizing light is removed. They have electrons that are easily excited to higher energy levels when they absorb light energy.
In phosphorescent materials—such as glow-in-the-dark objects—the excited electrons drop to a lower, but still excited intermediate level and stay there for a period of time before returning to their ground state (original energy level) and emitting the excess energy as visible light.