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Electrical switch used to transmit text messages in Morse code

A telegraph key or Morse key is a specialized electrical switch used by a trained operator to transmit text messages in Morse code in a telegraphy system. Keys are used in all forms of electrical telegraph systems, including landline (also called wire) telegraphy and radio (also called wireless) telegraphy. An operator uses the telegraph key to send electrical pulses (or in the case of modern CW, unmodulated radio waves) of two different lengths: short pulses, called dots or dits, and longer pulses, called dashes or dahs. These pulses encode the letters and other characters that spell out the message.

Since its original inception, the telegraph key's design has developed such that there are now multiple types of keys.

A straight key is the common telegraph key as seen in various movies. It is a simple bar with a knob on top and an electrical contact underneath. When the bar is pressed down against spring tension, it makes a closed electric circuit. Traditionally, American telegraph keys had flat topped knobs and narrow bars (frequently curved), while British telegraph keys had ball shaped knobs and thick bars. This appears to be purely a matter of culture and training, but the users of each are tremendously partisan. Straight keys have been made in numerous variations for over 150 years and in numerous countries. They are the subject of an avid community of key collectors. The straight keys used in wire telegraphy also had a shorting bar that closed the electrical circuit when the operator was not actively sending messages. This was to complete the electrical path to the next station so that its sounder would operate, as in the operator receiving a message from the next town. Although occasionally included in later keys for reasons of tradition, the shorting bar is unnecessary for radio telegraphy, except as a convenience when tuning the transmitter. The straight key is simple and reliable, but the rapid pumping action needed to send a string of dots (or dits as most operators call them) poses some significant drawbacks. Transmission speeds vary from 5 words (25 characters) per minute, by novice operators, up to about 30 words (150 characters) per minute by skilled operators. In the early days of telegraphy, a number of professional telegraphers developed a repetitive stress injury known as glass arm or telegrapher's paralysis. "Glass arm" or "telegrapher's paralysis" may be reduced or eliminated by increasing the side play of the straight key by loosening the adjustable trunnion screws. Such problems can be avoided by using a good technique.

In addition to the basic up and down telegraph key, telegraphers have been experimenting with alternate key designs from the beginning of telegraphy. Many are made to move side to side instead of up and down. Some of the designs, such as sideswipers (or bushwhackers) and semi automatic keys operate mechanically. Beginning in the mid 20th century electronic devices called "keyers" have been developed, which are operated by special keys of various designs generally categorized as single paddle keys (also called sideswipers), and double paddle keys (or "iambic" or "squeeze" keys). The keyer may be either an independent device that attaches to the transmitter in place of a telegraph key, or circuitry incorporated in modern amateurs' radios.

The first widely accepted alternative key was the sideswiper or sidewinder, sometimes called a cootie key or bushwhacker. This key uses a side to side action with contacts on both the left and right and the arm spring loaded to return to center; the operator may make a dit or dah by swinging the lever in either direction. A series of dits can be sent by rocking the arm back and forth. This first new style of key was introduced in part to increase speed of sending, but more importantly to reduce the repetitive strain injury affecting telegraphers, then popularly called "glass arm" or clinically "telegraphic paralysis". The side to side motion reduces strain, and uses different muscles than the up and down motion (called "pounding brass"). Nearly all advanced keys use some form of side to side action. The alternating action produces a distinctive rhythm or swing which noticeably affects the operator's transmission rhythm (known as his ‘fist’). Although the original sideswiper is now rarely seen or used, when the left and right contacts are electrically separated a sideswiper becomes a modern single paddle key (see below); likewise, a modern single lever key becomes an old style sideswiper when its two contacts are wired together.

A popular side to side key is the semi automatic key or bug, sometimes known as a Vibroplex key, after the company that first manufactured them. The original bugs were fully mechanical, based on a kind of simple clockwork mechanism, and required no electronic keyer. A skilled operator can achieve sending speeds in excess of 40 words per minute with a ‘bug’. The benefit of the clockwork mechanism is that it reduces the motion required from the telegrapher's hand, which provides greater speed of sending, and it produces uniformly timed dits (dots, or short pulses) and maintains constant rhythm; consistent timing and rhythm are crucial for decoding the signal on the other end of the telegraph line. The single paddle is held between the knuckle and the thumb of the right hand. When the paddle is pressed to the right (with the thumb), it kicks a horizontal pendulum which then rocks against the contact point, sending a series of short pulses (dits or dots) at a speed which is controlled by the pendulum’s length. When the paddle is pressed toward the left (with the knuckle) it makes a continuous contact suitable for sending dahs (dashes); the telegrapher remains responsible for timing of the dahs.

Like semi automatic keys, the telegrapher operates an electronic keyer by tapping a paddle key, swinging its lever(s) from side to side. When pressed to one side (usually left), the keyer electronics generate a series of dahs; when pressed to the other side (usually right), a series of dits. Keyers work with two different types of keys: Single paddle and double paddle keys. Like semi automatic keys, pressing the paddle on one side produces a dit and the other a dah. Single paddle keys are also called single lever keys or sideswipers, the same name as the older side to side key design they greatly resemble. Double paddle keys are also called "iambic" keys
or "squeeze" keys. Also like the old semi automatic keys, the conventional assignment of the paddle directions is that pressing a paddle with the thumb (for a right handed telegrapher, pressing the single paddle rightward, or for a double paddle key, pressing the left paddle rightwards to the center) creates a series of dits: The clockwork pendulum on old semi automatic keys needed the extra kick that the stronger thumb press provided. Pressing a paddle with the knuckle (hence swinging a single paddle leftward, or the right paddle on a double paddle key leftward to the center) creates a series of dahs. Left handed telegraphers sometimes elect to reverse the electrical contacts, so their left handed keying is a mirror image of standard right handed keying. Single paddle keys are essentially the same as the original sideswiper keys, with the left and right electrical contacts wired separately. Double paddle keys have one arm for each of the two contacts, each arm held away from the common center by a spring; pressing either of the paddles towards the center makes contact, the same as pressing a single lever key to one side. For double paddle keys wired to an "iambic" keyer, squeezing both paddles together makes a double contact, which causes the keyer to send alternating dits and dahs (or dahs and dits, depending on which lever makes first contact). Most electronic keyers include dot and dash memory functions, so the operator does not need to use perfect spacing between dits and dahs or vice versa. With dit or dah memory, the operator's keying action can be about one dit ahead of the actual transmission. The electronics in the keyer adjusts the timing so that the output of each letter is machine perfect. Electronic keyers allow very high speed transmission of code. Using a keyer in what's called "iambic" mode requires a key with two paddles: One paddle produces dits and the other produces dahs. Pressing both at the same time (a "squeeze") produces an alternating dit dah dit dah sequence, which starts with a dit if the dit side makes contact first, or a dah if the dah side connects first. An additional advantage of electronic keyers over semiautomatic keys is that code speed is easily changed with electronic keyers, just by turning a knob. With a semiautomatic key, the location of the pendulum weight and the pendulum spring tension and contact must all be repositioned and rebalanced to change the dit speed.

Keys having two separate levers, one for dits and the other for dahs are called dual or dual lever paddles. With a dual paddle both contacts may be closed simultaneously, enabling the "iambic" functions of an electronic keyer that is designed to support them: By pressing both paddles (squeezing the levers together) the operator can create a series of alternating dits and dahs, analogous to a sequence of iambs in poetry. For that reason, dual paddles are sometimes called squeeze keys or iambic keys. Typical dual paddle keys' levers move horizontally, like the earlier single paddle keys, as opposed to how the original "straight keys'" arms move up and down. Whether the sequence begins with a dit or a dah is determined by which lever makes contact first: If the dah lever is closed first, then the first element will be a dah, so the string of elements will be similar to a sequence of trochees in poetry, and the method could as logically be called "trochaic keying" (  ). If the dit lever makes first contact, then the string begins with a dit (  ). Insofar as iambic keying is a function of the electronic keyer, it is not correct, technically, to refer to a dual paddle key itself as "iambic", although this is commonly done in marketing. A dual paddle key is required for iambic sending, which also requires an iambic keyer. But any single or dual paddle key can be used non iambicly, without squeezing, and there were electronic keyers made which did not have iambic functions. Iambic keying or squeeze keying reduces the key strokes or hand movements necessary to make some characters, e. g. the letter C, which can be sent by merely squeezing the two paddles together. With a single paddle or non iambic keyer, the hand motion would require alternating four times for (dah dit dah dit ). The efficiency of iambic keying has recently been discussed in terms of movements per character and timings for high speed CW, with the author concluding that the timing difficulties of correctly operating a keyer iambicly at high speed outweigh any small benefits. Iambic keyers function in one of at least two major modes: Mode  and mode There is a third, rarely available mode .

Mode  is the original iambic mode, in which alternate dots and dashes are produced as long as both paddles are depressed. Mode  is essentially "what you hear is what you get": When the paddles are released, the keying stops with the last dot or dash that was being sent while the paddles were held.

Mode  is the second mode, which devolved from a logic error in an early iambic keyer. Over the years iambic mode  has become something of a standard and is the default setting in most keyers. In mode , dots and dashes are produced as long as both paddles are depressed. When the paddles are released, the keying continues by sending one more element than has already been heard. I. e., if the paddles were released during a dah then the last element sent will be a following dit; if the paddles were released during a dit then the sequence will end with the following dah. Users accustomed to one mode may find it difficult to adapt to the other, so most modern keyers allow selection of the desired mode.

A third electronic keyer mode useful with a dual paddle is the "Ultimatic" mode (mode ), so called for the brand name of the electronic keyer that introduced it. In the Ultimatic keying mode, the keyer will switch to the opposite element if the second lever is pressed before the first is released (that is, squeezed).

A single lever paddle key has separate contacts for dits and dahs, but there is no ability to make both contacts simultaneously by squeezing the paddles together for iambic mode. When a single paddle key is used with an electronic keyer, continuous dits are created by holding the dit side paddle ( ); likewise, continuous dahs are created by holding the dah paddle ( ). A single paddle key can non iambicly operate any electronic keyer, whether or not it even offers iambic functions, and regardless of whether the keyer iambically operates in mode , , or .

Simple telegraph like keys were long used to control the flow of electricity in laboratory tests of electrical circuits. Often, these were simple "strap" keys, in which a bend in the key lever provided the key's spring action. Telegraph like keys were once used in the study of operant conditioning with pigeons. Starting in the 1940s, initiated by B. F. Skinner at Harvard University, the keys were mounted vertically behind a small circular hole about the height of a pigeon's beak in the front wall of an operant conditioning chamber. Electromechanical recording equipment detected the closing of the switch whenever the pigeon pecked the key. Depending on the psychological questions being investigated, keypecks might have resulted in the presentation of food or other stimuli. Modern pigeon response keys are specially made switches but are still called "keys" due to their origins as telegraph keys.

With straight keys, side swipers, and, to an extent, bugs, each and every telegrapher has their own unique style or rhythm pattern when transmitting a message. An operator's style is known as their "fist". Since every fist is unique, other telegraphers can usually identify the individual telegrapher transmitting a particular message. This had a huge significance during the first and second World Wars, since the on board telegrapher's "fist" could be used to track individual ships and submarines, and for traffic analysis. However, with electronic keyers (either single or double paddle) this is no longer the case: Keyers produce uniformly "perfect" code at a set speed, which is altered at the request of the receiver, usually not the sender. Only inter character and inter word spacing remain unique to the operator, and can produce a less clear semblance of a "fist".