Resistors

To begin our examination of the development of the resistor and its most important characteristics, we will briefly review what you have already learned about resistors.

In most electrical circuits, there must be various levels of voltage and/or current at different points in the circuit. In a television set, for example, the tiny light that illuminates the station selection dial requires less voltage than is needed by the picture tube.

Resistors are devices that are used in electrical circuits to produce the desired current flows and voltages needed to make the circuit work properly. Resistors accomplish this objective by "resisting" the flow of electrons through the circuits, be it ac or dc.

You will also remember there are two basic types of resistors, fixed and variable. Fixed resistors can only provide a specific degree of resistance. Variable resistors, however, are adjustable for a range of resistance values.

Given this general knowledge of resistors, let us briefly consider the historical development of the variable resistor.

Early forms of variable resistor devices appeared in laboratories toward the end of the 19th century. These were large, bulky devices, like the one illustrated in Figure 2.21.

Figure 2.21

An insulated tray held an adjustable number of carbon and metal blocks. Major changes of resistance were accomplished by substituting metal blocks for some of the less conductive carbon ones. Metal terminal blocks could be placed anywhere in the stack to provide connections to circuits.

A later model (c.1929), shown in Figure 2.22, provided improvements such as wider adjustment range, good stability of resistance at high resistance values, and a greater wattage handling capacity due to cooling fins.

Figure 2.22

Another design consisted of a length of resistance wire and a sliding contact, as shown in Figure 2.23.

Figure 2.23

The total resistance between points A and B could be varied by using different materials for the wire. This is the forerunner of the variable resistor used today by the electronics industry.

There was one major limitation of the device in Figure 2.23: there were practical limits to the length of the wire that one could use. A solution needed to be found.

The need to get enough resistance in a practical amount of space led George Little to develop and patent a device that he called "Improvement in Rheostats or Resistance Coils" in 1871. Little wound insulated resistance wire around an insulated tube, or mandrel, in a tight spiral, or helix, as shown by the copy of his patent drawing in Figure 2.24.

Figure 2.24

The moving slider, or wiper arm, made contact with the resistance wire along a path where the insulation had been removed, thereby exposing a small portion of each wire.

In 1907, H. P. MacLagan was awarded a patent for a rotary rheostat. Figure 2.25 is a copy of his patent drawing. Mr. MacLagan had wound the resistance wire around a thin fiberboard card, and then formed the assembly into a circle. A wiper, attached to a center post, made contact with the resistance wire on the edge of the card.

Figure 2.25

During the years from 1920 to 1940, the explosive popularity of the radio created a demand for small electrical components. Especially, the need grew for small variable resistive devices, called potentiometers, for volume controls. Potentiometers used carbon or wire as resistive elements and were not that precise.

During World War II, the applications for electronic components grew dramatically. There was a need for cheaper and more accurate potentiometers. In 1945, Arnold Beckman invented the first commercially successful precision potentiometer.

In 1952, Marlan E. Bourns developed a miniature potentiometer for applications which required infrequent adjustment. A copy of his patent drawing is shown in Figure 2.26.

Figure 2.26

Bourns combined the advancing technologies of plastic molding and precision manufacturing to produce a potentiometer which was small, precise and inexpensive.

Since the introduction of the miniature adjustment potentiometer, improvements have led to better performance at a lower cost.