GB2192286A - Parallel protractor rule - Google Patents

Abstract

A parallel rule with a protractor comprises two plates 10 and 20 having parallel edges 11, 12, 21 and 22. These plates are joined by two pivoting arms 31 and 41 in such a way that the plates can be moved away from and towards each other while remaining parallel. The lower plate 10 of the ruler according to the invention is broader than the upper plate 20 and includes a central reference mark 15 in the middle of its top edge 11. Angular marks 18 connected with two graduations 1 and 2 run along the lower edge 12 and side edges 16 and 17 of the same plate 10. This makes it possible to measure an angle with the lower plate 10 alone, with no regard to the distance between the two plates 10 and 20. Nonslip feet are provided on the underside of the rule. The rule may be used for drawing or measuring bearings on a map, particularly for navigational purposes. <IMAGE>

Description

SPECIFICATION Parallel protractor rules This invention relates to parallel protractor rules. Parallel protractor rules are used by navigators to draw courses, bearings and position lines on nautical charts or maps. These rules have two rectangular plates of equal size that are joined to one another by means of two arms pivoted at each end with a respective one of the plates and spaced apart along the plates so that the plates can be displaced laterally away from one another whilst maintaining their edges parallel. In this way, one edge of the rule can be placed against a line and a second line drawn parallel to the first line at any distance from it as determined by the separation between the two plates. It is common for these rules to have angle markings on them so that the rule can be positioned at any desired angle with respect to a marking on the chart.The angle markings on previous parallel rules are produced along the outer edge of one plate whilst the datum point is marked on the outer edge of the other plate. The disadvantage of previous rules is that the angle markings can only be used when the two parts of the rule are closed against one another which prevents angles being set when the rule is expanded. There is also the disadvantage that wear of the rule or dirt on its abutting edges can prevent the two parts coming fully into contact and thereby lead to inaccuracies in the angle measurements. It is an object of the present invention to provide a parallel protractor rule that can be used to alleviate these disadvantages. According to the present invention there is provided a parallel protractor rule including two elongate plates each having first and second parallel straight edges and at least two coupling arms each of which is pivoted at opposite ends with respective plates such that the plates can be displaced laterally of the straight edges towards or away from one another whilst maintaining the edges of the two plates parallel with one another, one of the plates having a datum mark towards one edge and angle markings at points along the opposite edge of the same plate relating to the angle between the one edge and the points along the opposite edge subtended at the datum mark so that angles can be measured with the plates separated from one another. The widths of the two plates between their first and second parallel edges preferably differ from one another, the width of the plate carrying the datum mark and the angle markings being greater than the width of the other plate. In this way the overall width of the rule is kept to a minimum while that plate carrvinq the angle markings is of maximum width to ensure greater accuracy. The angle markings may be in the form of two scales for use respectively in measuring bearings in opposite directions. The datum marking is preferably carried towards an edge of one plate which abuts an edge of the other plate when the rule is closed. One or both plates may be provided with non-slip feet to prevent slipping of the rule on a chart. A parallel protractor rule in accordance with the present invention will now be described by way of example, with reference to the accompanying drawing which shows a plan view of the rule. The parallel protractor rule comprises two transparent plastics plates 10 and 20 of rectangular shape. Both plates 10 and 20 are 40cm long, but differ in width. The lower plate 10 in the drawing is 60mm wide while the upper plate 20 is only 35mm wide. The two plates 10 and 20 are coupled with one another by two metal arms 31 and 41 which are each about 9cm long. One end 32 and 42 of each arm 31 and 41 is pivoted with the lower plate 10 about axes normal to the plane of the plate so that the arms can be rotated in a plane parallel to that of the plates. The location of the pivots is close to the upper edge 11 of the lower plate 10 and spaced from one another along the rule by a distance of about 17cm. The other end 33 and 43 of each arm 31 and 41 is similarly pivoted with the upper plate 20 at points close to the lower edge 22 of the upper plate and spaced from one another along the rule by the same distance as the pivot points on the lower plate 10.This coupling enables the upper plate 20 to be displaced towards the lower plate 10 until the lower edge 22 of the upper plate contacts the upper edge 11 of the lower plate, or away from the lower plate by a distance limited by the length of the arms 31 and 41. As the two plates are moved towards or away from one another, the arms 31 and 41 are maintained parallel while the longitudinal edges 11 and 12, are also maintained parallel to the edges 21 and 22. The lower edge 22 of the upper plate 20, and the upper edge 11 of the lower plate 10, are square in profile so that they can be brought into intimate contact with one another when the rule is closed. The other edges of both plates 10 and 20 are bevelled to enable accurate reading of markings around the rule. Each plate 10 and 20 has a knob 14 and 24 respectively projecting vertically from their upper surface and located midway between the pivot points of the two arms 31 and 41. The upper edge 11 of the lower plate 10 has a single angular datum marking 15 located midway along its length. The lower edge 12 and the lateral end edges 16 and 17 of the upper plate carry angle marking 18 along them which relate to the angle between the upper edge 11 of the same plate subtended at the datum marking 15. The angle markings preferably comprise two scales alongside one another: an outer scale 1 ranging from 0 degrees to 180 degrees and an inner scale 2 ranging from 180 degrees to 360 degrees. Each scale is printed in a different colour for clarity. The smaller, upper plate 20 carries a distance scale 25 along its upper edge 21 graduated in millimetres and has engraved directional arrows 34 and 35 of the same two colours as the scales on the larger, lower plate 10. The underside surface of both plates 10 and 20 may have non-slip feet (not shown) to improve the grip of the rule on the surface of the chart. Conveniently, the feet may, for example, be provided by thin rubber rings around the pivot points of each arm 31 and 41. The rule is used with nautical or other charts in the conventional way to draw parallel bearing lines at any distance spacing from one another according to the separation between the two plates 10 and 20. The bearing lines may be drawn at any desired angle with respect to a line on the chart by using the angle markings on lower plate of the rule. A bearing or course may be read from a chart by placing the upper edge 21 of the upper plate 20 along the bearing line and then swinging the lower plate 10 away from the upper plate until the datum marking 15 lies against a convenient meridian on the chart. The bearing angle is then read off against the outer or inner scale 1 or 2.Where the bearing is in the direction indicated by the right-hand arrow 35 on the upper plate 20, the outer scale 1 is used; where the bearing is in the direction indicated by the left-hand arrow 34, the inner scale 2 is used. In previous parallel rules, the datum marking and angle markings are on different plates, thereby requiring the rule to be closed before an angle bearing can be set. In the present rule, by contrast, the angle bearings can be set with the rule in any expanded state, or with it closed, thereby considerably simplifying the procedure of setting bearing angles. Also, wear or contamination on the contacting edges of the two plates will not affect the accuracy of angle measurement of the present rule whereas, in previous rules, any failure of the two plates to close fully will reduce the accuracy of the angle measurement.

Claims (8)

1. A parallel protractor rule including two elongate plates each having first and second parallel straight edges and at least two coupling arms each of which is pivoted at opposite ends with respective plates such that the plates can be displaced laterally of the straight edges towards or away from one another whilst maintaining the edges of the two plates parallel with one another, wherein one of the plates has a datum mark towards one edge and angle markings at points along the opposite edge of the same plate relating to the angle between the one edge and the points along the opposite edge subtended at the datum mark so that angles can be measured with the plates separated from one another.

2. A parallel protractor rule according to Claim 1, wherein the widths of the two plates between their first and second parallel edges differ from one another.

3. A parallel protractor rule according to Claim 2, wherein the width of the plate carrying the datum mark and the angle markings is greater than the width of the other plate.

4. A parallel protractor rule according to any one of the preceding claims, wherein the angle markings are in the form of two scales for use respectively in measuring bearings in opposite directions.

5. A parallel protractor rule according to any one of the preceding claims, wherein the datum marking is carried towards an edge of one plate which abuts an edge of the other plate when the rule is closed.

6. A parallel protractor rule according to any one of the preceding claims, wherein one or both plates are provided with non-slip feet to prevent slipping of the rule on a chart.

7. A parallel protractor rule substantially as hereinbefore described with reference to the accompanying drawings.

8. Any novel feature or combination of features as hereinbefore described.