GB2264364A - Protractor - Google Patents

Abstract

A protractor comprises a main body (12) in the shape of a conventional half circular protractor from which a centre portion has been removed to leave an aperture (17). A rotary cursor (13) is mounted on the main body (12) and rotates about the centre of the protractor. In use an angle can be measured by aligning a base reference line (15) with one side of the angle, and aligning an edge (19) of the cursor (13) with the other side of the angle. Similarly an angle can be drawn by drawing along an edge (20) of the aperture (17), and along the edge (19) of the rotary cursory (13). In an alternative embodiment, the main body (12) extends around a full circle, and the cursor (13) is mounted by a central part disc-shaped portion which cooperates with a circular-shaped aperture in the main body (12). <IMAGE>

Description

PROTRACTOR The present invention relates to a protractor.

In known protractors there is a difficulty in measuring an angle when the angle is defined by lines shorter than the radius of the protractor, since then the lines defining the angle do not extend to the scale around the protractor. It is normally necessary to extend the lines concerned, if the angle is to be measured. Another difficulty arises in drawing lines at a required angle.

First the angle concerned needs to be marked off on the scale of the protractor, and then the line needs to be drawn after the protractor has been removed, by joining the mark from the scale, to a mark indicating the centre of the protractor.

It is an object of the present invention, at least in preferred embodiments thereof, to provide a protractor which overcomes or reduces these problems.

According to the present invention there is provided a protractor comprising a main body having a perimeter portion which extends around at least part of a circle, and which has a scale of angular measurements thereon, and a rotary cursor mounted on the main body for rotary movement about the centre of the said circle, the cursor having a reference feature which can be aligned with selected positions on the angular scale of the main body, whereby relative angular positions can be marked or measured by reference to the position of the cursor relative to the main body.

Preferably the cursor has a side edge extending along a radius of the circle, for alignment with a side of an angle to be measured, or for indicating an angle to be marked. Also preferably, the perimeter portion extends around an aperture in the main body, the cursor bering arranged to extend across the aperture, so that a line can be drawn along the edge of the cursor through the aperture in the main body.

In one arrangement the perimeter portion extends around substantially a semi-circle, and includes a diameter portion extending along a diameter of the said circle at the base of the semi-circle, the rotary cursor being mounted on the diameter portion at a centre portion thereof for rotary movement about the centre of the said circle.

In another arrangement, the perimeter portion extends around an entire circle. In such an arrangement, the perimeter portion may extend around a circular aperture in the main body, and the rotary cursor includes a circular centre portion engaged in the said circular aperture and rotatable within the aperture. Conveniently, the said centre portion of the rotary cursor extends around a portion of a circle bounded on one side by said side edge of said cursor, allowing the drawing of a line along the said side edge through the aperture in the perimeterportion.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1A shows a plan front view of a protractor embodying the present invention; Figure 1B shows a side view of the protractor of Figure lA, taken in the direction B in Figure lA; Figure 1C shows a side view of a rotary cursor of the protractor, taken along the direction C in Figure lA; Figure 1D shows the relationship between various edges and lines of the protractor shown in Figure lA; Figure 2A shows a plan view of the protractor of the proceeding figures with the rotary cursor removed; Figure 2B is a side view of the protractor taken in the direction B in Figure 2A; Figure 2C shows a section taken along the lines C-C in Figure 2A; Figure 2D shows a side view of the protractor taken in the direction D in Figure 2A;; Figure 3A shows a perspective view of the rotary cursor show in Figure 1A; Figure 3B shows is a plan view of the cursor of Figure 3A, taken from underneath; Figure 3C is a side view of the cursor taken in the direction C in Figure 33; Figure 3D is an end view of the cursor, taken in the direction D in Figure 3B; Figure 4 is a plan view of an alternative protractor embodying the invention; Figure 5A is a plan view of the main body of the protractor of Figure 4, with the cursor removed; Figure 5B is a section along the lines B-B in Figure 5A; Figure 6A is a plan view of the cursor shown. in Figure 4; and Figures 6B and 6C are side views of the cursor in the directions B and C shown in Figure 6A.

Referring firstly to Figure lA, there is shown a protractor 11 embodying the invention. The protractor comprises a main body 12 and a rotary cursor 13. The main body 12 comprises a perimeter portion 14 extending around a semi-circle together with a diameter portion 15 at the base of the semi-circle. An inner semi-circular portion of the main body of the protractor (shown cross hatched in Figure 2A at 16) is removed in manufacture, leaving an aperture 17 as shown in Figure 1A. Around the perimeter portion 14 is an angular scale 18 of conventional form.

The rotary cursor 13 has a shape in plan view approximately of a segment of a circle, and extends from the centre o of the circle of the perimeter portion 14, outwardly to and beyond the scale 18. As shown in Figures 1B and 1C, the rotary cursory 13 is mounted on the main body 12 for rotation about the centre o of the circle.

Various points and regions are indicated on Figure 1A by the letters a, b c, e, f, q, and h. Figure 1D shows the relationship between an angle cdh, and between an edge 19 of the cursor 13 and an edge 20 of the aperture 17; and an angle c o q defined by the edge 19 and a reference base line e o a f on the main body 12 of the protractor.

The edge 20 of the aperture 17 is made parallel to the reference base line e o f. The edge 19 of the cursor 13 is made to lie on a line passing through the centre o of the circle 10, and is made to extend out to the scale 18 on the perimeter portion 14. The outer portion of the edge 19 forms a reference feature which may be aligned with angular measurements on the scale 18.

Figure 2A shows the main body 12 of the protractor during manufacture, with a central cross hatched portion 16, which is cut away during manufacture. Figure 2C shows a section along the lines C-C in Figure 2A, after the portion 16 has been removed, and Figures 2B and 2D show side views along directions B and D respectively.

Referring again to Figure 1A, the cursor 13 is approximately "V" shaped. The portion a b of the rotor lines along an arch of an circle with radius greater than o e or o f, which is the radius of the perimeter portion 14. The cursor 13 has a circular centre portion 21 which has a column centre with centre o of the main body 12. The line bdo of the cursor 13 will rest along a radius of the protractor throughout movement of the cursor to any location. As shown in Figures 1A and 3A, the centre of the circular centre portion 21 of the cursor 13 is marked with a black dot on both sides. The dot coincides with the centre o which is the centre of the protractor's diameter.

Both the cursor 13 and the main body 12 are made of transparent material.

The operation of the embodiment will now be described.

The embodiment finds particular use to measure angles of small triangles, where an ordinary protractor produces difficulty because radial lines on the scale are marked only for every 10 interval, along the inner portion of the scale. If a line were to be drawn for ever 1" interval, the protractor would be congested with radial lines.

However in the embodiment of the invention, the side 19 along the b d of the cursor 13 functions as a rotating radius when the rotary cursor 13 is moved to any position.

To measure the angle of the triangle, the centre point o of the protractor is placed over the vertex of the triangle and the line o g is placed along one side of the triangle.

Next the rotary cursor 13 is rotated until the line b d (part of b d o) covers the line of the other side of the angle of the triangle to be measured. The value of the angle is read off from the protractor scale 18 where the reference feature comprising the outer portion of the edge 19 is aligned with a measurement of the scale. The embodiment enables the values of angle for small triangles to be measured easily and accurately without the need to extend the side of the angle to reach the scale of the protractor.

Another function of the protractor embodying the invention is to allow the drawing of an angle of a chosen value. For example, to draw an angle of 1600, the rotary cursor 13 is moved to the required reading on the scale, as shown in Figure IA The line c-d as shown in Figure 1A is drawn first, and then the line d-h is drawn. The angle required is now bounded by the lines c-d and d h. As shown in Figure lD, the angle c d h is equal to the angle c o a since d h and o q are two parallel lines. This is supported by the principle that two corresponding angles are equal in value.

Where an angle of small value such as 5 is to be drawn, a method of difference of angles is used, because the rotary cursor 13 would be at an extreme position either to the left or right when an angle of 5 is set up between the base line o q f, and the reference line o c b of the cursor. In such a position the normal drawing edge 19 of the rotary cursor would be obscured by the diameter portion 15 of the main body 12. Therefore, to draw an angle of, for example, 5", the side b c d of the rotary cursor 13 is moved to the 55" position on the scale 18, and a line is drawn c d. Next the rotary cursor 13 is moved until the line b c d is at the 50" position on the scale, and a line is again drawn along the edge 19 between c and d.The two lines are projected to meet to form an angle of 5".

In another use, a required angle can be drawn even more accurately, by first marking the points b and c onto the paper, and drawing the line d h onto the paper, and then extending the line bc to meet at d. This is more accurate because the edge 19 between c and d is not kept in touch with the paper onto which the angle is to be drawn, whilst the points b and c are in touch.

There will now be described with reference to Figures 4 to 6C, an alternative embodiment of the invention comprising a protractor 11 having a full 360" scale, and having a main body 12 in the shape of a complete circle.

Where components of the alternative embodiment correspond to components of the first described embodiment, they will be indicated by like reference numerals. Referring to Figure 4, the protractor 11 comprises a main body 12 and a rotary cursor 13. As shown in Figures 5A and 5B, the main body 12 has the shape of a full circular disc, with a centre portion removed to leave a centre circular aperture 17. As shown in Figure 5B, the inner edge of the main body 12 at the aperture 17 has a protruding flange 22.

As shown in Figures 6A to 6C, the rotary cursor comprises a sector-shaped portion 23 which abuts the flat main surface of the main body 12, and a central part discshaped portion 24. The centre portion 24 is disc-shaped apart from a 90C sector which is removed therefrom. One edge of the removed sector coincides with a reference edge 19 of the sector-shaped portion 23 of the rotary cursor.

Figures 6B and 6C show respectively side views of the rotary cursor 13, along directions B and C respectively in Figure 6A. As shown in Figures 6B and 6C, the central part disc-shaped portion 24 has a groove 27 around the periphery thereof, which, when assembled, engages with the projecting flange 22 on the main body 12. With reference to 6B, the manufacture of the protractor is achieved by forming the upper part of the central portion 24 (indicated at 28) separately from the lower part (indicated at 29). The upper part is placed in engagement with the main body, and the lower part 29 is then fastened to the upper part 28 by adhesive, thus capturing the central portion 24 in engagement with the main portion 12.

Referring again to Figure 4 which shows the assembled protractor, in use the edge 19 of the cursor 13 is in contact with, or very close to the paper, between the points o at the centre of the circle, and t at the inner edge of the main body 12. This allows a line to be drawn between o and t with the error due to parallax eliminated or greatly reduced.

The scale on the main body 12 is calibrated in the clockwise direction, but it can also be calibrated in an anti-clockwise direction, or in both directions When it is desired to draw an angle, the edge 19 of the cursor 13 is set to 0" and a line is drawn between o and t along edge 19. Next the cursor 13 is moved until the edge 19 reaches the required reading on the scale 18, and another line is drawn along the edge 19 between o and t to form the angle.

Referring again to Figures 6B and 6C, the sectorshaped portion 23 of the cursor rests lightly on the main body 12 of the protractor, and the central part disc-shaped portion 24 is almost touching the paper on which the line is to be drawn.

Claims (9)

1. A protractor comprising: a main body having a perimeter portion which extends around at least part of a circle, and which has a scale of angular measurements thereon, and a rotary cursor mounted on the main body for rotary movement about the centre of the said circle, the cursor having a reference feature which can be aligned with selected positions on the angular scale of the main body, whereby relative angular positions can be marked or measured by reference to the position of the cursor relative to the main body.

2. A protractor according to Claim 1 in which the cursor has a side edge extending along a radius of the circle, for alignment with a side of an angle to be measured, or for indicating an angle to be marked.

3. A protractor according to Claim 2 in which the perimeter portion extends around an aperture in the main body, the cursor being arranged to extend across the aperture, so that a line can be drawn along the edge of the cursor through the aperture in the main body.

4. A protractor according to any preceding claim in which the perimeter portion extends around substantially a semicircle, and includes a diameter portion extending along a diameter of the said circle at the base of the semi-circle, the rotary cursor being mounted on the diameter portion at a centre portion thereof for rotary movement about the centre of the said circle.

5. A protractor according to any of Claims 1 to 3 in which the perimeter portion extends around an entire circle.

6. A protractor according to Claim 5 in which the perimeter portion extends around a circular aperture in the main body, and the rotary cursor includes a circular centre portion engaged in the said circular aperture and rotatable within the aperture.

7. A protractor according to Claim 6 in which the said centre portion of the rotary cursor extends around a portion of a circle bounded on one side by said side edge of said cursor, allowing the drawing of a line along the said side edge through the aperture in the perimeter portion.

8. A protractor substantially as hereinbefore described with reference to Figures IA to 3D of the accompanying drawings.

9. A protractor substantially as hereinbefore described with reference to Figures 4 to 6C of the accompanying drawings.