Straight Line and its construction
From Math Images
Drawing a Straight Line 

Drawing a Straight Line
 The image shows the first planar linkage that drew a straight line without using a straight edge. Independently invented by a French army officer, CharlesNicolas Peaucellier and a Lithuanian (who some argue was actually Russian) mathematician Lipmann Lipkin, it had important applications in engineering and mathematics.^{[2]}^{[3]}^{[4]}
Introduction
What is a straight line? How do you define straightness? The questions seem silly to ask because they are so intuitive. We come to accept that straightness is simply straightness and its definition, like that of point and line, is simply assumed. However, why do we not assume the definition of circle? When using a compass to draw a circle, we are not starting with a figure that we accept as circular; instead, we are using a fundamental property of circles, that the points on a circle are at a fixed distance from the center. This page explores the answer to the question "how do you construct a straight line without a straight edge?"
What Is A Straight Line? A Question Rarely Asked.
Today, we simply define a line as a onedimensional object that extents to infinity in both directions and it is straight, i.e. no wiggles along its length. But what is straightness? It is a hard question because we can picture it, but we simply cannot articulate it.
Since we are dealing with plane geometry here, we define straight line as the curve of in Cartesian Coordinates.
Take a minute to ponder the question: "How do you produce a straight line?" Well light travels in straight line. Can we make light help us to produce something straight? Sure but does it always travel in straight line? Einstein's theory of relativity has shown (and been verified) that light is bent by gravity and therefore, our assumption that light travels in straight lines does not hold all the time. Well, another simpler method is just to fold a piece of paper and the crease will be a straight line. However, to achieve our ultimate goal (construct a straight line without a straight edge), we need a linkage and that is much more complicated and difficult than folding a piece of paper. The rest of the page revolves around the discussion of straight line linkage's history and its mathematical explanation.  
Image 1  Image 2 ^{[7]} 
The Quest to Draw a Straight Line
The Practical Need
James Watt's breakthrough
James Watt found a mechanism that converted the linear motion of pistons in the cylinder to the semi circular motion (that is moving in an arc of the circle) of the beam (or the circular motion of the flywheel) and vice versa. In this way, energy in the vertical direction is converted to rotational energy of the flywheel from where is it converted to useful work that the engine is desired to do. In 1784, he invented a three member linkage that solved the linearmotiontocircular problem practically as illustrated by the animation below. In its simplest form, there are two radius arms that have the same lengths and a connecting arm with midpoint P. Point P moves in a straight line while the two hinges move in circular arcs. However, this linkage only produced approximate straight line (a stretched figure 8 actually) as shown in Image 7, much to the chagrin of the mathematicians who were after absolute straight lines. There is a more general form of the Watt's linkage that the two radius arms having different lengths like shown in Image 6. To make sure that Point P still move in the stretched figure 8, it has to be positioned such that it adheres to the ratio.^{[15]}  
Image 6 ^{[16]}  Image 7 ^{[17]} 
The Motion of Point P
We intend to describe the path of so that we can show it does not move in a straight line (which is obvious in the animation). More importantly, this will allow us to pinpoint the position of using certain parameters we know, such as the angle of rotation or one coordinate of point . This is awfully crucial in engineering as engineers would like to know that there are no two parts of the machine will collide with each other throughout the motion. In addition, you can use the parametrization to create your own animation like that in Image 7.
Algebraic Description
We see that moves in a stretched figure 8 and will tend to think that there should be a nice closed form of the relationship of the and coordinates of like that of the circle. After this section, you will see that there is a closed form, at least theoretically, but it is not "nice" at all. 
Image 8 
We know coordinates and because they are fixed. We know coordinates and because they are fixed. Hence suppose the coordinates of are and coordinates of are . We also know the length of the bar. Let . Suppose that at one instance we know the coordinates of as , then will be on the circle centered at with a radius of . Since is on the circle centered at with radius . Then the coordinates of have to satisfy the two equations below.
Now, since we know that is on the circle centered at with radius , the coordinates of have to satisfy the equation . Therefore, the coordinates of have to satisfy the three equations below.
Now, expanding the first two equations we have,
Subtract Eq. 2 from Eq. 1 we have, Substituting and rearranging we have,
Hence Now, we can manipulate Eq. 3 to get an expression for , i.e. . Next, we substitute back into Eq. 1 and will be able to obtain an expression for , i.e. . Since , we have expressions of and in terms of and . Say point has coordinates , then and which will yield In the last step we substitute ,, Eq. 5 and Eq. 6 back into Eq. 4 and we will finally have a relationship between and . Of course, it will be a messy closed form but we could definitely use Mathematica to do the maths. The point is, there is no nice algebraic form for that figure 8, though it has closed form and that is why we have to find something else.

Parametric Description
Alright, since the algebraic equations are not agreeable at all, we have to resort to the parametric description. To think about, it may be more manageable to describe the motion of using the angle of ratation. As a matter of fact, it is easier to obtain the angle of rotation than knowing one of 's coordinates. 
Image 9 
We will parametrize the with the angle in conformation of most para [...] We will parametrize the with the angle in conformation of most parametrizations of point.
Now let . Then using cosine formula, we have As a result, we can express and as
Since , and being the coordinates of point , we can find in terms of . Furthermore, Therefore, Hence,
Now, is parametrized in term of and .

Image 10 ^{[19]} 
Watt's Secret
Another reason we parameterized is that Watt did not simply used that three bar linkage shown in Image 6 and Image 7. Instead he used something different. To understand that, our knowledge of the parameterizaion of is crucial. Imitations were a big problems back in those days. When filing for a patent, James Watt and other inventors had to explain how their devices worked without revealing the critical secrets so that others could easily copy them. As shown in Image 10, the original patent illustration, Watt illustrated his simple linkage on a separate diagram on the upper left hand corner but try looking for it on the engine illustration itself. Can you find it at all? That is Watt's secret. This is the equivalent of telling you by using the principle of 1+1 makes 2 you could get 34 x 45; the crucial step in understanding (and to make the engine work smoothly in Watt's case) is avoided. What he had actually used on his engine was the modified version of the basic linkage as show in Image 11.
 
Image 11  Image 12 
Well, it is easy enough. Refer to Image 12. Well, it is easy enough. Refer to Image 12. . Furthermore . Therefore, . We now have the parameterization of point and and Watt's secret is eventually cracked.

The First Planar Straight Line Linkage  PeaucellierLipkin Linkage
Image 13 ^{[20]}  Anyway, mathematicians and engineers had being searching for almost a century to find the solution to a straight line linkage but all had failed until 1864 when a French army officer Charles Nicolas Peaucellier came up with his inversor linkage. Interestingly, he did not publish his findings and proof until 1873, when Lipmann I. Lipkin, a student from University of St. Petersburg, demonstrated the same working model at the World Exhibition in Vienna. Peaucellier acknowledged Lipkin's independent findings with the publication of the details of his discovery in 1864 and the mathematical proof. Taimina

Image 14  
Let's turn to a skeleton drawing of the PeaucellierLipkin linkage in Image 14. It is constructed in such a way that and . Furthermore, all the bars are free to rotate at every joint and point is a fixed pivot. Due to the symmetrical construction of the linkage, it goes without proof that points , and lie in a straight line. Construct lines and and they meet at point .
Since shape is a rhombus and Now,
Therefore, Let's take a moment to look at the relation . Since the length and are of constant length, then the product is of constant value however you change the shape of this construction.  
Image 15  
Refer to Image 15. Let's fix the path of point such that it traces out a circle that has point on it. is the extra link pivoted to the fixed point with . Construct line that cuts the circle at point . In addition, construct line such that .
Since,
Moreover . Therefore constant, i.e. the length of (or the xcoordinate of w.r.t ) does not change as points and move. Hence, point moves in a straight line. ∎^{[21]} 
Inversive Geometry in PeaucellierLipkin Linkage
As a matter of fact, the first part of the proof given above is already sufficient. Due to inversive geometry, once we have shown that points , and are collinear and that is of constant value. Points and are inversive pairs with as inversive center. Therefore, once moves in a circle that contains , then will move in a straight line and vice versa. ∎ See Inversion for more detail.
PeaucellierLipkin Linkage in Action
Image 16 
The new linkage caused considerable excitement in London. Mr. Prim, "engineer to the House", utilized the new compact form invented by H.Hart to fit his new blowing engine which proved to be "exceptionally quiet in their operation." In this compact form, , and . Point and are fixed pivots. In Image 16. F is the inversive center and points , and are collinear and is of constant value. 
Image 17 
Mr. Prim's blowing engine used for ventilating the House of Commons, 1877. The crosshead of the reciprocating air pump is guided by a Peaucellier linkage shown in the middle of Image 17. Prim's machine was driven by a steam engine.^{[22]} 
Hart's Linkage
After the PeaucellierLipkin Linkage was introduced to England in 1874, Mr. Hart of Woolwich Academy ^{[23]} devised a new linkage that contained only four links which is the blue part as shown in Image 18. The next part will prove that point is the inversion center with and collinear and constant. When point is constrained to move in a circle that passes through point , then point will trace out a straight line. See below for proof. 
Image 18 
We know that
As a result, Draw line , intersecting at point . Consequently, points are collinear Construct rectangle
For , we then have . Further, let's define where We finally have which is what we wanted to prove. 
Other Straight Line Mechanism
Image 19  Image 20  Image 21 ^{[24]} 
There are many other mechanisms that create straight line. I will only introduce one of them here. Refer to Image 19. Consider two circles and with radius having the relation . We roll inside without slipping as show in Image 20. Then the arch lengths . Voila! and point has to be on the line joining the original points and ! The same argument goes for point . As a result, point moves in the horizontal line and point moves in the vertical line. In 1801, James White patented his mechanism using this rolling motion. It is shown in Image 21 ^{[25]}.  
Image 22  
Interestingly, if you attach a rod of fixed length to point and and the end of the rod will trace out an ellipse as seen in Image 22. Why? Consider the coordinates of in terms of , and . Point will have the coordinates .
Now, whenever we see and together, we want to square them. Hence, and . Well, they are not so pretty yet. So we make them pretty by dividing by and by , obtaining and . Voila again! and this is exactly the algebraic formula for an ellipse. ^{[26]} 
ConclusionThe Take Home Message
We should not take the concept of straight line for granted and there are many interesting, and important, issues surrounding the concepts of straight line. A serious exploration of its properties and constructions will not only give you a glimpse of geometry's all encompassing reach into science, engineering and our lives, but also make you question many of the assumptions you have about geometry. Hopefully, you will start questioning the flatness of a plane, roundness of a circle and the nature of a point and allow yourself to explore the ordinary and discover the extraordinary.
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About the Creator of this Image
KMODDL is a collection of mechanical models and related resources for teaching the principles of kinematicsthe geometry of pure motion. The core of KMODDL is the Reuleaux Collection of Mechanisms and Machines, an important collection of 19thcentury machine elements held by Cornell's Sibley School of Mechanical and Aerospace Engineering.
Related Links
Additional Resources
 http://kmoddl.library.cornell.edu/model.php?m=244
 http://dlxs2.library.cornell.edu/cgi/t/text/textidx?c=math;cc=math;view=toc;subview=short;idno=Kemp009
 http://kmoddl.library.cornell.edu/tutorials/04/
 http://www.howround.com/
 http://en.wikipedia.org/wiki/Wikipedia:Citing_sources
Notes
 ↑ Wikipedia (Linkage (mechanical))
 ↑ Bryant, & Sangwin, 2008, p. 34
 ↑ Kempe, 1877, p. 12
 ↑ Taimina
 ↑ Wikipedia (Cartesian coordinate system)
 ↑ Wikipedia (Linkage (mechanical))
 ↑ Weisstein
 ↑ Bryant, & Sangwin, 2008, p. 18
 ↑ Bryant, & Sangwin, 2008, p. 18
 ↑ Wikipedia (Steam Engine)
 ↑ Bryant, & Sangwin, 2008, p. 1821
 ↑ Bryant, & Sangwin, 2008, p. 1821
 ↑ Bryant, & Sangwin, 2008, p. 1821
 ↑ Bryant, & Sangwin, 2008, p. 1821
 ↑ Bryant, & Sangwin, 2008, p. 24
 ↑ Bryant, & Sangwin, 2008, p. 23
 ↑ Wikipedia (Watt's Linkage)
 ↑ Wikipedia (Closedform expression)
 ↑ Lienhard, 1999, February 18
 ↑ Wikipedia (Peaucellier–Lipkin linkage)
 ↑ Bryant, & Sangwin, 2008, p. 3336
 ↑ Ferguson, 1962, p. 205
 ↑ Kempe, 1877, p. 18
 ↑ Bryant, & Sangwin, 2008, p.44
 ↑ Bryant, & Sangwin, 2008, p.4244
 ↑ Cundy, & Rollett, 1961, p. 240
References
 Bryant, John, & Sangwin, Christopher. (2008). How Round is your circle?. Princeton & Oxford: Princeton Univ Pr.
 Cundy, H.Martyn, & Rollett, A.P. (1961). Mathematical models. Clarendon, Oxford : Oxford University Press.
 Henderson, David. (2001). Experiencing geometry. Upper Saddle River, New Jersey: Prentice hall.
 Kempe, A. B. (1877). How to Draw a straight line; a lecture on linkage. London: Macmillan and Co..
 Taimina, D. (n.d.). How to Draw a Straight Line. Retrieved from The Kinematic Models for Design Digital Library: http://kmoddl.library.cornell.edu/tutorials/04/
 Ferguson, Eugene S. (1962). Kinematics of mechanisms from the time of watt. United States National Museum Bulletin, (228), 185230.
 Weisstein, Eric W. Great Circle. Retrieved from MathWorldA Wolfram Web Resource: http://mathworld.wolfram.com/GreatCircle.html
 Wikipedia (Steam Engine). (n.d.). Steam Engine. Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Steam_engine
 Wikipedia (Watt's Linkage). (n.d.). Watt's Linkage. Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Watt%27s_linkage
 Wikipedia (Cartesian coordinate system). (n.d.). Cartesian coordinate system. Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Cartesian_coordinate_system
 Wikipedia (Linkage (mechanical)). (n.d.). Linkage (mechanical). Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Linkage_(mechanical)
 Wikipedia (Closedform expression). (n.d.). Closedform expression. Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Closedform_expression
 Lienhard, J. H. (1999, February 18). "I SELL HERE, SIR, WHAT ALL THE WORLD DESIRES TO HAVE  POWER". Retrieved from The Engines of Our Ingenuity: http://www.uh.edu/engines/powersir.htm
 Wikipedia (Peaucellier–Lipkin linkage). (n.d.). Peaucellier–Lipkin linkage. Retrieved from Wikipedia: http://en.wikipedia.org/wiki/Peaucellier%E2%80%93Lipkin_linkage
Leave a message on the discussion page by clicking the 'discussion' tab at the top of this image page.
colspan="2"Today, we simply define a line as a onedimensional object that extents to infinity in both directions and it is straight, i.e. no wiggles along its length. But what is straightness? It is a hard question because we can picture it, but we simply cannot articulate it.
In Euclid's book Elements, he defined a straight line as "lying evenly between its extreme points" and as having "breadthless width." This definition is pretty useless. What does he mean by "lying evenly"? It tells us nothing about how to describe or construct a straight line.
So what is a straightness anyway? There are a few good answers. For instance, in the Cartesian Coordinates , the graph of is a straight line as shown in Image 1. In addition, the shortest distance between two points on a flat plane is a straight line, a definition we are most familiar with. However, it is important to realize that the definitions of being "shortest" and "straight" will change when you are no longer on flat plane. For example, the shortest distance between two points on a sphere is the the "great circle" as shown in Image 2.
Since we are dealing with plane geometry here, we define straight line as the curve of in Cartesian Coordinates.
For more comprehensive discussion of being straight, you can refer to the book Experiencing Geometry by David W. Henderson.
Take a minute to ponder the question: "How do you produce a straight line?" Well light travels in straight line. Can we make light help us to produce something straight? Sure but does it always travel in straight line? Einstein's theory of relativity has shown (and been verified) that light is bent by gravity and therefore, our assumption that light travels in straight lines does not hold all the time. Well, another simpler method is just to fold a piece of paper and the crease will be a straight line. However, to achieve our ultimate goal (construct a straight line without a straight edge), we need a linkage and that is much more complicated and difficult than folding a piece of paper. The rest of the page revolves around the discussion of straight line linkage's history and its mathematical explanation. 
align="center"