In many circuits several condensers so connected that their rotors can be turned simultaneously with a single shaft control are used. These are called gang condensers and are indicated symbolically by dotted lines leading from the center of each condenser to a single horizontal line (see Fig. 17).

Resistances, which in one form or another play an equally important part in radio circuits, are represented by zigzag lines (see Fig. 17). Like condensers, resistances also take on two forms; being either fixed or variable. If the resistor is of the fixed type, it is symbolized by a plain zigzag line. If, like the variable condenser, it is adjustable, the zigzag line is supplemented by an arrow drawn through its center at an angle.

Variable resistances are used in two forms known as rheostats and potentiometers. In the rheostat, one connection is made to the movable resistance arm while the second is made to one end of the resistance winding. The potentiometer has three connections. One is made to the movable arm, another to one end of the resistance, and the third to the other end of the resistance.

Since the operation of the modern radio receiver or transmitter is based on the vacuum tube, its symbol will be encountered every time the amateur picks up a radio book or blueprint. It is represented in circuit drawings in a number of ways depending on the particular type and the number of internal elements it uses.

Simplest of all common radio tubes, is the three-clement filament variety. It consists mainly of a glass

envelope or bulb, a tiny wire filament, a coil of wire known as the grid, and a flattened cylinder of metal called the plate. The conventionalized symbol for this type of tube consists of a heavy circle representing the glass bulb enclosing a hairpin-shaped line indicating the filament, a zigzag line the grid, and a square the plate (see Fig. 17).

As the radio tube becomes more complicated so does its symbol. If it is of the heater variety, having an independently heated cathode instead of a filament, the filament lines are replaced by a U-shaped heater symbol and a line is drawn over the heater in the form of a hook to represent the cathode. The grid and plate are indicated as before.

If to this symbol, a cage of zigzag lines is added so that it encloses the square symbolizing the plate, it indicates that the tube is of the screen grid variety and the cage represents the screen. Symbols for still other tubes having more than one grid contain additional zigzag lines to indicate these parts.

Although it is impossible for the amateur to learn, the exact symbol for every type of tube that is manufactured, since more than eighty different varieties are being made, he can, by memorizing the basic symbols for grids, screen grids, plates, heaters, and filaments, figure out the connections indicated by al-most any drawing.

In simplified diagrams, the beginner may find that the tubes are indicated by a slightly different set of symbols. But this should give no trouble since the filaments in this case are generally drawn V-shaped,

the plates T-shaped, and the grids and screens as dotted lines.

In picturing the countless other radio parts that go to make up radio circuits, the symbols follow the same simple rules. A coil of wire, for instance, is represented by a spiral line drawn to look like a pulled-out spring. Two such coils drawn one on top of the other indicate a coupled radio tuning coil such as the usual type of plug-in coil employed in many simple receivers, A single coil, supplemented by the letters RFC indicates a radio-frequency choke.

If the part consists of one or more coils of wire wound on a metal core, the core is represented by three or four parallel lines drawn close together either through the coils, between them, or alongside of them. As an example, study the symbols for the audio trans-former, power transformer, and filter choke.

Electric meters are indicated by small circles drawn around letters to indicate the type. For instance, a voltmeter is represented by a circle enclosing the let-ter V, an ammeter a circle enclosing the letter A, and a milliammeter the letters MA (see Fig. 17).

Simplest of all symbols, but at times the most con-fusing, are the conventions used to indicate whether or not two wires that cross each other in the diagram are connected. There are two systems of symbols used to distinguish wires that join from wires that merely cross. One way is to make a looped curve in one of the lines (wires) to indicate that it is bent over the other line (wire) and does not touch it. The other method is to let the lines cross but to consider that

they are not connected unless a dot is placed at the junction point.

In some radio diagrams you will find that a combination of the two systems is used, but a brief study of the lines generally will reveal what type of symbols the designer used.

Headphones and microphones (see Fig. 17) are symbolized merely by simplified pictures while loudspeaker drawings will vary with the type of speaker. A magnetic speaker having two connections, generally is represented by a spiral drawn around a single line projecting from the apex of a triangle drawn to simulate the cone. The dynamic speaker, on the other hand, consisting of a voice coil and a field coil is represented by two spirals, one representing the field winding and the other indicating the voice coil.

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