Reverse salient
The reverse salient refers to a backward bulge in the advancing line of a military front. The term has been commonly used to analyze military campaigns in the First World War, such as in the Battle of Verdun
Battle of Verdun
The Battle of Verdun was one of the major battles during the First World War on the Western Front. It was fought between the German and French armies, from 21 February – 18 December 1916, on hilly terrain north of the city of Verdun-sur-Meuse in north-eastern France...

, where opposing military forces created uneven sections in respective battle lines. The significance of the reverse salient lies in the idea that in its presence, the forward progress of a military front is slowed down or halted. This is because opposing forces threaten to break through the military line along this weak, bowed back section, subsequently requiring effort in bringing the reverse salient section forward, in line with the rest of the military front.

In his seminal book, Networks of power: Electrification in western society, 1880-1930, Thomas P. Hughes
Thomas P. Hughes
Thomas Parke Hughes is an American Historian of Technology. He is an emeritus professor of history at the University of Pennsylvania and is a visiting professor at MIT and Stanford.He received his Ph.D. from the University of Virginia in 1953....

 introduces the concept in the analysis of technological systems, whereby the reverse salient refers to a component of the system that, due to its insufficient development, prevents the technological system in its entirety achieving its targeted development.

Technological systems and their evolution

Technological systems may refer to a hierarchically
A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...

 nested structure of technological parts, whereby the system
System is a set of interacting or interdependent components forming an integrated whole....

 is seen as a composition of interdependent
Interdependence is a relation between its members such that each is mutually dependent on the others. This concept differs from a simple dependence relation, which implies that one member of the relationship can function or survive apart from the other....

 sub-systems that are themselves systems comprising further sub-systems . In this manner, the holistic system as well as its properties is seen to be synthesis
In general, the noun synthesis refers to a combination of two or more entities that together form something new; alternately, it refers to the creating of something by artificial means...

ed through the sub-systems that constitute it. Technological systems may also be seen as socio-technical systems
Sociotechnical systems
Sociotechnical systems in organizational development is an approach to complex organizational work design that recognizes the interaction between people and technology in workplaces. The term also refers to the interaction between society's complex infrastructures and human behaviour...

 that contain, in addition to technical sub-systems, social sub-systems, such as the creators and users of technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, as well as overseeing regulatory bodies. In both perspectives, technological systems are seen to be goal-seeking, therefore evolving towards objectives .

Hughes proposes that technological systems pass through certain phases during the system’s evolution. The first of these phases sees the invention and development of the system, owed greatly to the efforts of inventors and entrepreneur
An entrepreneur is an owner or manager of a business enterprise who makes money through risk and initiative.The term was originally a loanword from French and was first defined by the Irish-French economist Richard Cantillon. Entrepreneur in English is a term applied to a person who is willing to...

s, such as Thomas Edison
Thomas Edison
Thomas Alva Edison was an American inventor and businessman. He developed many devices that greatly influenced life around the world, including the phonograph, the motion picture camera, and a long-lasting, practical electric light bulb. In addition, he created the world’s first industrial...

 in the development of the electric technological system. The second stage is the era of technological transfer from one region or society to others, for example, the dissemination of Edison’s electric system from New York City to other cities such as London and Berlin. The third phase of systemic evolution is marked by a period of system growth and expansion when the technological system strives to improve its performance, for instance with respect to economic outcomes or output
Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...

 efficiency. In this phase, the system is dependent on the satisfactory evolution of all its components’ performances. The development of the technological systems is therefore reliant on the reciprocated and interdependent cause and effect processes
Process (science)
In science, a process is every sequence of changes of a real object/body which is observable using the scientific method. Therefore, all sciences analyze and model processes....

 amongst social and technical components, and may be described as co-evolutionary, where the balanced co-evolution of system components carries significance in establishing desired system progress. Subsequently, a sub-system which evolves at a sufficient pace contribute positively to the collective development, while a sub-system which does not develop sufficiently prevents the technology system achieving its targeted development. Hughes names these problematic sub-systems reverse salients.

Reverse salients in technological system evolution

A reverse salient is the inverse of a salient
Salients, re-entrants and pockets
A salient is a battlefield feature that projects into enemy territory. The salient is surrounded by the enemy on three sides, making the troops occupying the salient vulnerable. The enemy's line facing a salient is referred to as a re-entrant...

 that depicts the forward protrusion along an object’s profile or a line of battle. Hence, reverse salients are the backward projections along similar, continuous lines. The reverse salient subsequently refers to the sub-system that has strayed behind the advancing performance frontier of the system due to its lack of sufficient performance. In turn, the reverse salient hampers the progress or prevents the fulfillment of potential development of the collective system. In line with the socio-technical standpoint, reverse salients can be technical elements such as motors
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...

 and capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s of an electric system, or social elements such as organizations or productive units.

Because reverse salients limit system development, the further development of the system lies in the correction of the reverse salient, where correction is attained through incremental or radical innovation
Innovation is the creation of better or more effective products, processes, technologies, or ideas that are accepted by markets, governments, and society...

s. The reverse salient denotes a focusing device, in the words of Nathan Rosenberg
Nathan Rosenberg
Nathan Rosenberg is an American economist specializing in the history of technology. He earned his PhD from the University of Wisconsin in 1955, and has taught at Indiana University , the University of Pennsylvania , Purdue University , Harvard University , the University of...

, for technological system stakeholders
Stakeholder theory
The stakeholder theory is a theory of organizational management and business ethics that addresses morals and values in managing an organization. It was originally detailed by R...

, which strive to remove it through innovation. It is possible that the reverse salient is not able to be corrected within the bounds of the existing technological system through incremental innovations
Innovation is the creation of better or more effective products, processes, technologies, or ideas that are accepted by markets, governments, and society...

. Consequently, radical innovations
Innovation is the creation of better or more effective products, processes, technologies, or ideas that are accepted by markets, governments, and society...

 may be needed to successfully correct the reverse salient. However, radical innovations can lead to the creation of new and different technological systems, as witnessed in the emergence of the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 system that successfully overcame the problem of low cost electricity distribution, which the direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 system could not.

Hence, the reverse salient is a useful concept for analyzing technological system evolution, because often the analysis of technological systems centers on the factors that limit system development. More than technical components, these factors may also be social components. Subsequently, reverse salients may be more applicable in certain contexts to denote system performance hindrance than similar or overlapping concepts such as bottleneck
A bottleneck is a phenomenon where the performance or capacity of an entire system is limited by a single or limited number of components or resources. The term bottleneck is taken from the 'assets are water' metaphor. As water is poured out of a bottle, the rate of outflow is limited by the width...

 and technological imbalance or disequilibrium.

The reverse salient refers to an extremely complex situation in which individuals, groups, material forces, historical influences, and other factors have idiosyncratic, causal forces, and in which accidents as well as trends play a part. On the contrary, the disequilibrium concept suggests a relatively straightforward abstraction of physical science. Additionally, while the reverse salient and bottleneck concepts share similarities and have been used interchangeably in particular contexts, the reverse salient often refers to the sub-system that not only curbs the performance or output of the collective system but also requires correction because of its limiting affect. This is not necessarily the case with bottlenecks, which are geometrically too symmetrical and therefore do not represent the complexity of system evolution. For instance, a particular system’s output performance may be compromised due to a bottleneck sub-system but the bottleneck will not require improvement if the system’s present output performance is satisfactory. If, on the other hand, a higher level of performance would be required of the same system, the bottleneck may emerge as a reverse salient that holds the system back from attaining that higher output performance.

Reverse salient examples

While numerous studies illustrate technological systems that have been hampered by reverse salients, the most seminal work in this field of study is that of Hughes, who gives a historical account of the development of Edison’s direct-current electric system. In order to supply electricity within a defined region of distribution, sub-systems such as the direct current generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 were identified as reverse salients and corrected. The most notable limitation of the direct-current system was, however, its low voltage transmission distance, and the resulting cost of distributing electricity beyond a certain range. To reduce costs, Edison introduced a three-wire system to replace the previously installed two-wire alternative and trialed different configuration of generators, as well as the usage of storage batteries. These improvements however did not correct the reverse salient completely. The satisfactory resolution of the problem was eventually provided by the radical innovation of the alternating current system.

Since Hughes' seminal work, other authors have also provided examples of reverse salients in different technological systems. In the ballistic missile
Ballistic missile
A ballistic missile is a missile that follows a sub-orbital ballistic flightpath with the objective of delivering one or more warheads to a predetermined target. The missile is only guided during the relatively brief initial powered phase of flight and its course is subsequently governed by the...

 technological development, where the systemic objective has been to increase missile accuracy, MacKenzie has identified the gyroscope
A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. In essence, a mechanical gyroscope is a spinning wheel or disk whose axle is free to take any orientation...

 sub-system as a technical reverse salient. Takeishi and Lee have argued that music copyright
Copyright is a legal concept, enacted by most governments, giving the creator of an original work exclusive rights to it, usually for a limited time...

 managing institutions have acted as a social reverse salient in the evolution of the mobile music
Mobile music
Mobile music is music which is downloaded or streamed to mobile phones and played by mobile phones. Although many phones play music as ringtones, true "music phones" generally allow users to stream music or download music files over the internet via a WiFI connection or 3G cell phone connection. ...

 technology system in Japan and Korea, where the objective was to proliferate mobile music throughout the end-user market. And further, Mulder and Knot, see the development of the PVC (polyvinyl chloride
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...

) plastic technology system to have been sequentially hampered by several states of reverse salience, including: difficulty to process PVC material, quality of manufactured products, health concerns for individuals exposed to effluent from PVC manufacturing facilities, and finally the carcinogen
A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer. This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes...

ic nature of vinyl chloride.

Analytical measure of reverse salience

The magnitude of reverse salience emerges as an informative parameter
Parameter from Ancient Greek παρά also “para” meaning “beside, subsidiary” and μέτρον also “metron” meaning “measure”, can be interpreted in mathematics, logic, linguistics, environmental science and other disciplines....

 in technological systems analysis as it signifies not only the technological disparity between sub-systems but also the entire system’s limited level of performance. Notwithstanding its importance, the literature studying technological system evolution has remained limited in terms of analytical tools that measure the state of reverse salience. Dedehayir and Mäkinen have subsequently proposed an absolute
Absolute value
In mathematics, the absolute value |a| of a real number a is the numerical value of a without regard to its sign. So, for example, the absolute value of 3 is 3, and the absolute value of -3 is also 3...

performance gap measure of reverse salience magnitude. This measure evaluates the technological performance differential between the salient sub-system (i.e. the advanced sub-system) and the reverse salient sub-system at a particular point in time. In turn, by evaluating a series of performance differentials over time, the performance gap measure helps reflect the dynamics of change in the evolving technological system through changing reverse salience magnitude.
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