28. Medium Frequency. 1361

Medium frequency (MF) refers to radio frequencies (RF) in the range of 300 kHz to 3 mHz. Frequencies immediately below MF are denoted low frequency (LF), and the next higher frequencies are known as high frequency (HF).

Propagation at MF is usually via ground waves. MF ground-wave propagation depends on the ionosphere's D-layer. When heavily ionized, such as during the day, especially in summer, and more especially at times of high solar activity, this atmospheric layer can be electronically noisy and absorptive of MF waves. For this reason, many MF transmitters, whether for broadcast or communication purposes, need to be high-powered to punch the power through the noise and losses.

Late at night, especially in winter months, and particularly at times of low solar activity, the ionospheric D-layer can virtually disappear. When this happens, MF radio waves can easily be received hundreds or even thousands of miles away.

This can be very useful for long-distance communication, but can have the opposite effect in many other cases. For example, due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasters provided they transmit several hundred miles apart. At nights of good MF propagation, distant stations may appear superimposed onto local ones causing interference.

29. Early History Of Power Plugs and Sockets. 1465

But they were not always safe: some people AC power plugs and sockets are devices for connecting electrically operated gadgets to the power supply.

An electric plug connects mechanically to a matching socket. Usually plugs are movable connectors and sockets are fixed to equipment.

When electricity was first introduced into the household, it was primarily used for lighting. At that time, many electricity companies operated a split-tariff system where the cost of electricity for lighting was lower than that for other purposes. This led to portable appliances (such as vacuum cleaners, electric fans, and hair driers) being connected to light bulb sockets.

However, as electricity became a common method of lighting houses and operating labour-saving appliances, a safe means of connection to the electric system was needed.

The first original two pin electrical plug and socket were invented by Harvey Hubbell and patented in 1904.

Other manufacturers adopted the Hubbell pattern and by 1915 they were widespread, although in the 1920s and even later, household and light commercial equipment was still powered through cables connected with Edison screw-base adapters. This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performancegot an electric shock.

By the 1920^th as the need for safe installations became apparent, grounded three-contact systems were used in most industrial countries.