A partial analysis of features present in various models as an aid to determining age
When I first started collecting compasses, I knew nothing about these superb instruments. I knew a bit about compasses; baseplate compasses for map reading, and lensatic compasses for back-country navigation, etc., but these big things I kept running into called "Bruntons" were a mystery to me. I began researching them, I quickly learned, and as I learned, I fell in love with these precision devices. Originally patented in 1894 by Canadian-born Aspen, Colorado geologist David W. Brunton, the Brunton Pocket Transit (or simply "Brunton" to many professionals) is a very high quality compass and clinometer. It has a unique mix of practical features that have made it an enduring design. Some of the features include:
In the words of the inventor: "... I have provided means for taking both horizontal and vertical angles and for obtaining clinometer readings, and have made the instrument sufficiently small and light to be carried in the vest pocket." There are instruments which are more accurate, and there are instruments which are more portable, and there are instruments which have all the functions of a Brunton and more, but the unique blend of multiple functions in a small form-factor that could be clipped on a belt or tucked in a pocket made this instrument an instant hit. The Brunton Pocket Transit has been in continual production by various manufacturers for nearly 110 years. The design has been honed and refined, and new materials have been utilized along the way, but still the vision of a compact multi-function instrument has remained true. Today Bruntons are used by geologists, paleontologists, archaeologists, and other scientists and engineers who need a survey instrument that can go where others can not.
WARNING: Beware of 'Brass Brunton' compasses.
There are a large number of replica "Brass" versions now for sale, some for as little as $10. These are intended for use as display pieces, and not as functional instruments. I have seen some of these misrepresented on online auction sites, and yet sell for over $200. Many of these sellers are sincere, and think they have an antique functional instrument.
Almost all Brunton cases were manufactured from either aluminum or a composite polycarbonate resin material. See my page on these reproductions for more information.
I used to think that all brass pocket transits were fakes. However, I have received e-mail from Bud Uzes, a Sacramento, CA area surveyor, author of a book on the history of surveying in America, and a collector of survey instruments, who states that both he and a well-known dealer in scientific antiquities, Dale Beeks, have seen high-quality brass Brunton pocket transits. These guys have forgotten more about antique instruments than I will ever know, so I trust their word. However, these items must be very rare, as I have only heard these two reports about them, and have never even seen a picture. I suspect that any brass units were made as commemorative pieces, or perhaps made-to-order for a specific client.
Why wasn't brass widely used? Primarily because of weight. Brass (an alloy of copper and zinc) has a specific gravity of 8.4-8.7. Aluminum has a specific gravity of 2.55-2.80. Using these numbers, we can estimate that 6 ounces of aluminum has the same volume as 18 ounces of brass ( ( 6 / 2.8 ) * 8.4 = 18 ). Since the primary purpose of a pocket transit is to go where full-sized instruments can't, carrying around more than a pound of brass defeats the purpose of a portable 'vest pocket' device.
Brass is also more subject to corrosion, especially in damp environments such as exist inside mines.
The cheap brass reproductions seen everywhere should not be confused with high-quality copies of the Brunton design manufactured by other makers, such as this one by Keuffel & Esser.
The remainder of this page deals with features of Brunton Pocket Transits, and almost exclusively the features found in two production lines: Ainsworth and Brunton. While other manufacturers certainly made Pocket Transits, these two companies produced the vast majority of the Pocket Transits in circulation.
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Early Brunton Pocket Transits manufactured by the William Ainsworth company varied in the type of engraving used on the transits, and this is perhaps one of the most reliable methods of dating an early Ainsworth unit. The Ainsworth company offered various features as options. For example, it was possible to order a Pocket Transit with crossed long levels instead of the later style mixed long and round levels as late as 1929, possibly much later. Obviously if there is a patent date engraved on the unit, then the unit can not be older than that date.
The earliest unit that I have information on is #105, which is hand engraved on the mirror lid as follows:
There are some name name changes regarding the Ainsworth company which may also be helpful in determining a date. See the serial number comparison table, especially the Ainsworth section. The information that I have is that "Wm. Ainsworth" became "Wm. Ainsworth & Son" around 1899, and "Wm. Ainsworth & Sons" in about 1905. There is some controversy over when the changes were made, and I have not seen an Ainsworth Brunton with the maker name as "Wm. Ainsworth & Son" Given the number of units produced with the older company name I tend to lean toward the later date as being correct for the company name change.
Another feature of the engraving which reliably indicates date of manufacture is the patent dates. See the links page for links directly to patent images at the U.S. Patent and Trademark Office. The following patent numbers and dates are known:
Another date indicator in markings is the Trademark claim. The "D.W. Brunton's" trademark was issued on June 4, 1946 to the William Ainsworth Company, therefore any transits with "Registered Trademark" are post 1946.
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There is some variation in needle types that can be used to assist dating a unit.
The earliest units have a straight vertical bar steel needle with no arrowheads. The North end of the needle may be painted white. See this image on the Gemmary website as an example. Note the long, slender appearance of the needle, which is in stark contrast to the thick stamped-metal look of the needle on this cheap reproduction.
The first change was to add a white-painted arrowhead on the north arm of the needle. This appears to be accomplished by putting a 1/4 twist in the end of the needle. See this image on the Gemmary website as an example. Note once again the slender needle.
The next change was to add an arrowhead at the south arm of the needle. This was most likely done to support taking backsights with the small leaf sight. I believe that the addition of the southern arrowhead corresponds to the addition of the small folding sight, which was patented in 1913. At about this time, the design of the needle changed from a vertical bar to a horizontal bar.
Throughout these changes, the needles were long, slender, and very graceful. In units balanced for the Northern Hemisphere, a weight consisting of a copper wire may be present around the shaft of the southern arm of the needle. This is due to the fact that the magnetic lines of force that move the needle do not run parallel to the surface of the earth in most parts of the world. Instead they intersect the surface at an angle. The angle that they intersect the surface causes the needle to not ride completely level in on the pivot, and the weight counteracts this imbalance. In units balanced for the southern hemisphere, the weight will appear on the northern arm. It appears from photographs that some early versions of the needle used the length of the arms to balance the needle.
From this point the design is fairly stable for several years, until the arrival of the rare earth magnets (See section below).
The needles for AlNiCo Bruntons have a carriage in the center that holds the magnets. They are usually bent in the center, so that the magnets and the needle tips ride at different levels. They are bent different in directions based on the model. Early AlNiCo carriages ride very high in the capsule, and the needles drop down from the magnet carriage to ride next to the graduated ring. This is continued, with some modifications and improvements in the modern "Conventional" model. With the "International" model, the situation is reversed, and the AlNiCo carriage rides very low in the capsule, and the needle bends upwards toward the tips.
The newest form is the NdFeB model needle (Com-Pro, Geo-Pro), which is straight, lays across the top of the magnet disk, and has an index line and "N" and "S" printed on the tips.
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There are three types of magnets found used in Bruntons; Steel, AlNiCo, and NdFeB.
Bruntons with AlNiCo magnets have a needle that holds two long AlNiCo magnets at it's center. Depending on the model and date of manufacture, there may or may not be an induction cup around the magnets. AlNiCo Models with 'international balance' have a carriage which is very low to the face of the compass, and the needle is bent in the middle into a shallow U-shape. The base of the U is where the pivot and magnets are, and the tips bend up toward the crystal before flattening out at a level even with the azimuth ring. The pivot itself is housed in a 'tower' sitting on top of the needle, between the two AlNiCo magnets. This design lowers the center of gravity so that the compass will be usable in a wider range of latitudes. Note: This does not permit a unit balanced for use in the northern hemisphere to operate in the Southern Hemisphere.
AlNiCo magnets first became available in the 1940s, and I believe that they were not commercially available until after WW2. They may have been used in M2 compasses during WW2. Generally, however, Bruntons with AlNiCo magnets are Post-WW2.
AlNiCo models were produced by Ainsworth both with and without induction damping. AlNiCo models were also produced by the Brunton Company as well as others.
Bruntons with NdFeB magnets have a straight needle that rides on top of the magnet, which looks like circular black disk with a flattened edge.
NdFeB magnets were not commercially available until the 1980s, and were very expensive until the mid to late 80s. Generally, NdFeB Bruntons will date to post 1985. All, or nearly all NdFeB models were produced by the Brunton Company. As far as I know, there were no NdFeB models produced by Ainsworth or any other manufacturer.
This feature is found only on units with AlNiCo or NdFeB magnets. Some early AlNiCo units lack this feature.
The induction damping system consists of a copper cup that surrounds the rare earth magnets very closely. This cup can be seen on modern AlNiCo and NdFeB Bruntons. The cup may or may not be painted black. Most of the units with induction damping are painted black, but some unit with unpainted bright copper induction cups are in circulation.
The movement of the powerful magnets induces eddy currents in the copper. These currents produce their own magnetic field which interferes with the magnetic field of the permanent magnets, and this has a braking effect on the motion of the magnets. The faster the motion of the needle, the greater the braking effect.
The beauty of this system is the magnets essentially brake themselves, and the faster they are moving, the greater the braking effect. Yet when the needle is settling down and moving slowly, the braking effect is almost non-existent. This is superior to the venerable fluid damping system found on many compasses.
As far as I know, induction damping is used on all NdFeB model Bruntons, and on all AlNiCo model Bruntons of modern manufacture. Some early Ainsworth Bruntons did not have induction damping. I do not know an exact date after which all AlNiCo Ainsworth Bruntons had damping. Ainsworth had historically offered various features as options, so it is difficult to use the presence or absence of a particular feature as a dating method. Ainsworth is also reported to have retrofitted units.
There are at least three versions of the dial; quads, degrees, and mils. I have never seen a pocket transit graduated in grads or gons, but I have been told that such units exist. Note that all pocket transits made since the beginning of the 20th century have "reversed" or "direct reading" compass points and graduations. This is due to their primary use as survey instruments instead of navigation instruments. (see below)
The difference between the two systems is subtle, but important. The primary use of a survey compass is to read a bearing from one point to another point. Using a needle-type navigation compass (such as a Silva Ranger) to read a bearing, you would first align the compass with the object ("shoot the bearing"), then rotate the bezel until the needle was 'boxed'. The bearing is then read off of the bezel ring where it is aligned with the lubber's line.
Using a direct-read survey compass such as a Brunton, you shoot the bearing, and then read the bearing at the tip of the needle where it aligns with the graduated ring. Fewer steps means less chance of error.
Card-type compasses resolve this by having the scale on the card, which rotates with the needle. However, they are not as easily read, and in general are not graduated to one degree as the Brunton is.
In this graduation method, North and South are marked '0', and East and West are marked '90'. The size of each graduation is a degree. This is the "quad" ("quadrants") system, because you need four "quadrants" of 90 degrees to add up to the full 360 degree circle.
The quad system (also called "Quadrant" or "Surveyor's" system) has been used for property surveys and property descriptions on deeds and legal documents for many years. At the time that Brunton was developing his instrument, it was the standard in geology as well. Today you will find both 'quad' and 'degree' systems in use. The notation for a "quad" compass is slightly different than for a 'degree' compass. In a 'degree' or 'azimuth' compass, the direction South-West would be read as 225 degrees. In the quad system it is read as "South 45 degrees West", ("S 45d W" in notes).
It may seem silly to base angles on this but it's use in the military is adopted for purely practical reasons. Because of the relationship between a milliradian and pi, 1 mil is equal to the range divided by 1000. So a mil roughly equals 1 yard horizontal distance at 1000 yards range, 2 yards at 2000 yards, 1 meter at 1 kilometer, etc. It makes adjusting fire right and left less error-prone, especially in battlefield situations. Note: The U.S. Military has rounded the 6283 milliradians in a complete circle up to 6400, thus a U.S. Military 'mil' equal 0.05625 degrees. Some other countries use 6000 mils per full circle, or 0.006 degrees per mil.
The U.S. Military version of the pocket transit is the M2 compass. Older versions were produced by various manufacturers. Today the M2 is made exclusively by the Brunton company, and is the same compass as the ComPro, with an Olive Drab colored case and graduated in mils instead of degrees or quads.
To my knoweldge, nearly all military issue M2 compasses were manufactured with a scale graduated in mils. However, I have seen at least one older M2 with a 'degree' scale. Because repair services were an important income stream, both Ainsworth and the Brunton Company would replace a azimuth ring graduated in mils with one graduated in either degrees or quads.
It has recently been brought to our attention that very early Ainsworth Bruntons had a fixed or factory-set declination. A transit, in the collection of Dr. Peter von Bitter, in the 600ís serial number range has no declination screw. Number 105, in the collection of Jerry Mohlman also has no declination screw, and is factory set for a declination of 15 degrees east. Number 904, also in the collection of Jerry Mohlman, has a declination screw.
The declination adjustment screw does not appear in the original patent, but appears in the 1912 patent.
As more data are gathered in this area, a new column will most likely be added to the serial number table.
Nearly all genuine Bruntons, regardless of the manufacturer, have bodies that are made out of either aluminum or composite resin (plastic). The composite body is a recent addition, appearing in the mid 1980s. The aluminum bodies appear in different finishes; polished aluminum as in the 1894 original or the updated 1914 version, painted black as in the 1926 Keuffel & Esser version, or painted gray or olive drab as in the M2 version.
Pictures of Composite-bodied Bruntons can be recognized by the sinuses in the top lid, around the mirror. Aluminum-bodied Bruntons lack these sinuses, plus the corners appear noticeably sharper.
Early units in the Ainsworth line only had the single long sight. D. W. Brunton added the second, short sight in 1913 to facilitate taking backsights. This procedure allows the surveyor to detect and correct for errors due to observation errors, or errors induced by a local mineral deposit. Nearly all Ainsworth Bruntons have a short sight which folds flat on the outside of the case. In 1926 Keuffel & Essor introduced their improvements to the Ainsworth design, which included the round level, and the innovation of having the small sight fold down inside the case instead of outside. In order to accomodate the small sight, a notch was introduced at the base of the long sight. This can be seen on the Brunton Evolution page.
Ainsworth did produce some units with the inside folding sight, but not until late in their production.
Bruntons with an inside-folding sight can often be detected by looking for the wide notch at the base of the long sight.
 Please note that this drawing differs most Bruntons as manufactured. Although very early Bruntons were manufactured with the faces marked as above, sometime between serial numbers 254 and 614, the face was reversed, and the star representing North was moved next to the folding sight, "S" was moved to the mirror hinge side, and "E" and "W" were reversed. This is how all Bruntons were manufactured after that point. See the serial number comparison table for more information.
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