In 1668, Francesco Redi, an Italian scientist, conducted an experiment with jars of meat. This experiment was noteworthy because it disapproved the hypothesis of spontaneous generation.
He placed fresh meat in each of two separate jars. One of the jars was left open and the other was covered with a cloth. A few days later, the open jar contained maggots while the covered jar contained no maggots, although there were maggots on the exterior surface of the cloth covering the jar.
Wi-Fi technology has its origins in a 1985 ruling by the U.S. Federal Communications Commission that released the bands of the radio spectrum at 900 megahertz (MHz), 2.4 gigahertz (GHz), and 5.8 GHz for unlicensed use by anyone. Technology firms began building wireless networks and devices to take advantage of the newly available radio spectrum, but without a common wireless standard the movement remained fragmented, as devices from different manufacturers were rarely compatible. Eventually, a committee of industry leaders came up with a common standard, called 802.11, which was approved by the Institute of Electrical and Electronics Engineers (IEEE) in 1997. Two years later a group of major companies formed the Wireless Ethernet Compatibility Alliance (WECA, now the Wi-Fi Alliance), a global nonprofit organization created to promote the new wireless standard. WECA named the new technology Wi-Fi. Subsequent IEEE standards for Wi-Fi have been introduced to allow for greater bandwidth. The original 802.11 standard allowed a maximum data transmission rate of only 2 megabits per second (Mbps); 802.11n, introduced in 2007, has a maximum rate of 600 Mbps.
Under the IEEE Wi-Fi standards, the available frequency bands are split into several separate channels. These channels overlap in frequency, and therefore Wi-Fi uses channels that are far apart. Within each of these channels, Wi-Fi uses a “spread spectrum” technique in which a signal is broken into pieces and transmitted over multiple frequencies. Spread spectrum enables the signal to be transmitted at a lower power per frequency and also allows multiple devices to use the same Wi-Fi transmitter. Because Wi-Fi signals are often transmitted over short distances (usually less than 100 metres [330 feet]) in indoor environments, the signal can reflect off walls, furniture, and other obstacles, thus arriving at multiple time intervals and causing a problem called multipath interference. Wi-Fi reduces multipath interference by combining three different ways of transmitting the signal (in a method developed by Australian engineer John O’Sullivan and collaborators). The popularity of Wi-Fi has grown steadily. Wi-Fi allows local area networks (LANs) to operate without cables and wiring, making it a popular choice for home and business networks. Wi-Fi can also be used to provide wireless broadband Internet access for many modern devices, such as laptops, smartphones, tablet computers, and electronic gaming consoles. Wi-Fi-enabled devices are able to connect to the Internet when they are near areas that have Wi-Fi access, called “hot spots.” Hot spots have become common, with many public places such as airports, hotels, bookstores, and coffee shops offering Wi-Fi access. Some cities have constructed free citywide Wi-Fi networks. A version of Wi-Fi called Wi-Fi Direct allows connectivity between devices without a LAN. Radiotelegraphy, radio communication by means of Morse Code or other coded signals. The radio carrier is modulated by changing its amplitude, frequency, or phase in accordance with the Morse dot-dash system or some other code. At the receiver the coded modulation is recovered by an appropriate demodulator and the code groups are converted into the corresponding symbols. In many instances the symbols are generated by a computer and modem rather than with a manual telegraph key.
Stomata,
the pore like openings in a leaf.
gases pass though them.
