Cathode

A **cathode** is an elechode through which electric current flows //out of// a polarized electrical device. The direction of electric current is, by convention, opposite to the direction of electron flow. Therefore the electrons flow into the polarized electrical device and out of, for example, the connected electric circuit. Mnemonic; CCD (Cathode Current Departs). Cathode polarity is not always negative. Although positively charged cations always move towards the cathode (hence their name) and negatively charged anions move away from it, cathode polarity depends on the device type, and can even vary according to the operating mode. In a device which consumes power, the cathode is negative, and in a device which provides power, the cathode is positive:
 * In a discharging battery or a galvanic cell the cathode is the positive terminal since that is where the current flows out of the device (see drawing). This outward current is carried internally by positive ions moving from the electrolyte to the positive cathode (chemical energy is responsible for this "uphill" motion). It is continued externally by electrons moving inwards, negative charge moving one way constituting positive current flowing the other way. For example, the [|Daniell galvanic cell] 's copper electrode is the positive terminal and the cathode.
 * In a recharging battery, or an electrolytic cell, the cathode is the negative terminal, which sends current back to the external generator. For example, reversing the current direction in a [|Daniell galvanic cell] would produce an electrolytic cell, [|[1]] where the copper electrode is the positive terminal and the anode.

Cathode rays
By the 1870s, British physicist [|William Crookes] and others were able to evacuate tubes to a lower pressure, below 10−6 atm. These were called Crookes tubes. Faraday had been the first to notice a dark space just in front of the cathode, where there was no luminescence. This came to be called the "cathode dark space", "Faraday dark space" or "Crookes dark space". Crookes found that as he pumped __more__ air out of the tubes, the Faraday dark space spread down the tube from the cathode toward the anode, until the tube was totally dark. But at the anode (positive) end of the tube, the glass of the tube itself began to glow. What was happening was that as more air was pumped from the tubes, the electrons could travel farther, on average, before they struck a gas atom. By the time the tube was dark, most of the electrons could travel in straight lines from the cathode to the anode end of the tube without a collision. With no obstructions, these low mass particles were accelerated to high velocities by the voltage between the electrodes. These were the cathode rays. When they reached the anode end of the tube, they were travelling so fast that, although they were attracted to it, they often flew past the anode and struck the back wall of the tube. When they struck atoms in the glass wall, they excited their orbital electrons to higher energy levels, causing them to [|fluoresce]. Later researchers painted the inside back wall with fluorescent chemicals such as [|zinc sulfide], to make the glow more visible. Cathode rays themselves are invisible, but this accidental fluorescence allowed researchers to notice that objects in the tube in front of the cathode, such as the anode, cast sharp-edged shadows on the glowing back wall. In 1869, German physicist [|Johann Hittorf] was first to realize that something must be travelling in straight lines from the cathode to cast the shadows. [|Eugen Goldstein] named them//cathode rays//.

Cathodes used for field electron transmission in vacuum tubes are called cold cathodes. Heated electrodes or hot cathode, frequently called filaments are much more common. Most radios and television sets prior to the 1970s used filament-heated-cathode electron tubes for signal selection and processing; to this day, a hot cathode forms the source of the electron beam(s) in cathode ray tubes in many television sets and computer monitors. Hot electron emitters are also used as the electrodes in fluorescent lamps and in the source tubes of x-ray machines.

Cathodes are also found in __batteries__ When dealing with batteries, don’t think in terms of anode and cathode; think in terms of positive terminal and negative terminal. When dealing with semiconductor diodes, don’t worry about anode and cathode; think in terms of P-doped side and N-doped side. The general rule is: Anode means current //into// the black box and cathode means current //out from// the black box. Zener diodes give rise to an execrable exception that should be avoided like the plague. There is abundant evidence that even people who call themselves experts cannot keep the anode/cathode terminology straight. In any practical situation, there is always a way to figure out how to hook things up without a deep understanding of anode versus cathode. In almost all situations, it is better to avoid the terms anode and cathode. There are better ways to say what needs to be said. Constructive suggestion: it is better to talk about the current (rather than the electrode). It is better to talk about what the current is //doing// (rather than what the electrode “is”).