 Let me introduce myself. I am Dr. Prakash Saunke, working as associate professor in Mechanical Engineering Department of Waltzian Institute of Technology, Sulapu. Today, in continuation with our meteorology series, we shall be discussing mechanical comparators. In mechanical comparators today, we shall be discussing Sigma Comparator. So, students will be able to explain principle construction and working of Sigma Comparator and will describe merits and demerits of Sigma Comparator as well as mechanical comparators. Sigma Comparator. Every comparator is a precision instrument used to compare the dimensions of given component with the standard and measure the deviation from the standard. Sigma Comparator is one of the mechanical comparator and consists of dial and pointer as a display unit. The displacement is sensed through a plunger or stylus. So, if we refer figure one, say we will find yes, this is the Sigma Comparator. So, this is the dial and pointer that is a display unit and say this is the stylus or plunger. The mechanism which is used for magnification is placed in a metal casing normally of a circular shape and Sigma Comparator is mounted on some stand or suitable base. So, you can see this. So, this is the metal casing inside which say there will be the mechanism and say this is mounted on some kind of stand, some kind of stand. So, this is how the outward appearance of Sigma Comparator. Now, let us try to understand the construction of Sigma Comparator. So, let us see what are different components. So, one of the important component is the plunger. It is a cylindrical spindle which is in direct contact with the workpiece and it is a notch at its center. So, if we see the figure of mechanism of Sigma Comparator you will see. So, this is the plunger, this is the plunger. So, it is a cylindrical one that is the sensing element, sensing element. Now, after the plunger say there is a slit diaphragm. It consists of two flat steel springs and they hold a plunger in place. So, they also act as a spring. So, let us see their functioning later on. Then, one more important element is knife edge. It is mounted at the notch of the plunger and transmits the motion of a plunger. Again, if we see the figure of mechanism you will see this is the knife edge. So, this is the notch in the plunger and say this is mounted placed in the say I mean notch. Now, this knife edge say it bears on the moving block. So, bears upon the moving block. So, moving block. So, it I mean is in direct contact of the knife edge and this moving block is a part of cross-strip hinge cross-strip hinge. If we see the figure. So, this is the say moving block. This is the moving block and this knife edge is resting upon say this particular moving block and this is the cross-strip hinge as you can see. So, this is a cross. So, these are the four pieces four pieces say cross-strip hinge. It consists of four flat steel springs arranged at right angles and having a pivot at the center at the center. Pivot is very efficient and sensitive to small angular motion. So, if we see here. So, this is the central pivot and this is the cross-strip hinge. So, one end is this moving block another is the fixed block fixed block and say this pivot is very sensitive. So, it can say since the seven motion and this strip hinges. So, they will be moving with respect to each other. Now, one more important element is y-shaped arm. So, it is also called as a forked arm. So, it is a light metal forked arm having a y-shape which consists which connects the cross-strip to driving drum. If we see the mechanism. So, here you can see. So, this is the y-shaped arm forked arm and it is on one end it is connected to this cross-strip hinge. As the cross-strip hinge say moves say due to movement of this moving block say then that motion will transmitted to say this y-shaped and say this y-shaped transmits the motion to this driving drum. So, we shall see again the working. Driving drum it is a small drum which supports the spindle carrying a pointer. Then there is a metal ribbon it is a thin phosphor bronze ribbon fashioned at two ends of the forked arm and wrapped around a driving drum. So, if we see the figure you can see. So, this is the phosphor bronze ribbon and it is I mean connected at two ends of the forked arm and it is wrapped around the driving drum. So, that driving drum is pressed. So, it is in say this phosphor bronze is in tension. Then pointer and scale. So, these are the other same in two parts. Now, let us try to understand the working of sigma comparator. The plunger is placed initially in direct contact with the standard. So, normally that plunger the stylus so that end is touched to the standard and then it is slightly loaded to move upward. So, plunger is slightly loaded to move upward. So, if we see here so this plunger so that will be slightly loaded in upward direction. So, it will be pressed up in upward direction. So, such that stand is adjusted. Now, tell me why to do this initial loading of plunger. Can you tell yes think yes loading is done so that if any work piece is of smaller dimension than the standard the plunger has to move downwards from the zero position to sense the deviation. So, if say it is not loaded there will not be any scope to move the plunger in downward direction when smaller component is pressed. So, initially it is loaded upward and at this position zero setting of the scale is done. You understood why loading is to be done. Now, the standard will be replaced by the work piece which are which is to be checked one by one. If there is deviation in the dimension of the work piece plunger will move vertically upward or downwards. Just see here if when that piece is pressed here initially for standard it is slightly loaded and adjusted to zero. Now, work pieces will be set there one by one. If the work piece is of higher larger dimension then it will push the push the plunger upwards. If it is of smaller dimension then due to this slit diaphragm which acts as a spring so that will push the plunger downwards. Initially it is loaded so there is scope for moving towards the downward direction. So, this is how that deviation is sensed. Now, this deviation the motion of the plunger is transmitted to the knife edge which in turn presses the moving block of cross strip hinge. So, if we see so this motion is transmitted to this knife edge and this knife edge if it is moving upwards so this knife edge will move upwards or downwards say and say then say that will press the moving block accordingly. This moving block say will transmit the motion to this cross strip hinge. Cross strip hinge will move around the pivot around the pivot and the motion will be passed to the Y shaped arm which will rotate the drum through a ribbon. So, if we see here say this motion will transmit to Y shaped arm and Y shaped arm will now reciprocate like this like this and say like this and so that will rotate say this particular passport ribbon move and the ribbon will in turn move this particular driving drum. When the driving drum rotates the spindle mounted on it so that will also rotate and say that motion will be transmitted to the pointer so which will move on the scale. So, magnification is obtained in two stages ratio of effective length of Y arm L and distance of the knife edge from the point pivot A. So, provided so that provides the first stage of magnification where second stage of magnification is due to ratio of length of pointer L and radius of driving drum R. So, total magnification will be equal to L by A into small L by R. So, magnification the range is normally 300 to say 5000, 5000. Taken the references I have taken the references from textbook of meteorology by M. Mahajan, Danpat Rai and Company Private Limited I am thankful to you and I thank you also.