The different fixed resistors
Resistors with a well-defined value are called fixed resistors. According to the manufacturing and the materials, there are three types of fixed resistors.
1- layer resistors
Also called film resistors, they consist essentially of a porcelain or steatite support on which a thin layer of high electrical resistance (graphite, silica or carbon) has been deposited. The deposit is made by applying a varnish. The value of the resistance depends on the thickness of the deposited layer and the material, the thinner the layer, the higher the value of the resistance. The thickness of the layer is difficult to control. To obtain high-value resistors, a thin layer is deposited which is spirally shaped by means of a lathe which engraves a spiral groove on the porcelain (or soapstone). This layer takes the form of a spirally wound ribbon. The value of the resistance is thus increased because the current will have more way to go. The layer constituting the resistance is protected by an insulating varnish. The resistant material that is in contact with the copper wires attached to the body of the resistor. With the spiraling we obtain high value resistances.
2- the agglomerated resistances
There are two groups of agglomerated resistors: conductive body and conductive layer. The conductive body resistors consist of a paste of carbon metal oxides (or carborundum mixed with agglomerating substances), which is given a cylindrical shape. The two connections are sealed in the body of the resistor and the whole is protected by an insulating material. The value of a conductive body resistor is determined by the dimensions of the cylindrical portion between the two connections. Conductive layer resistors are more common. They are formed of a mixture of carbon and a binder material constituting a layer deposited on a small glass tube. The two connections penetrate this glass tube and are in contact with the outer resistant layer. Everything is contained in a plastic envelope. These resistors are small because the arrangement of the connections allows good cooling of the resistant layer by conduction through the glass.
3- resistance to wire or wire
These resistors are designed to be traversed by high value currents. They are formed of an insulating support on which is wound a high electrical resistance wire. The wires are in constantan, manganese or still in nichrome. There are several types of wire resistance. As an example: The resistant wire may be uniformly wound around a porcelain tube. The ends of the resistant wire are connected to tinned copper wire. Everything is then covered with protective insulation. The resistant wire may be wrapped around a bakelite wafer. The resistant wire is then connected to metal tongues (of various shapes) attached to the ends. The resistant yarn can also be wrapped around a mineral support resistant to high temperatures. The resistant element is firmly attached to the ends. The whole is covered with an insulating plastic coating. The resistant wire may be wrapped around a mineral support, all placed in a square soapstone envelope and sealed with a special cement. These resistors can withstand higher current intensities better than other coil resistors.
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