A Late Spanish Vernier Sextant

26 12 2012

In a recent exchange of e-mails with David Monton Farrioli of Breda in north-eastern Spain he drew my attention to an illustration in a book on navigation written in Spanish (Astronomia Nautica y Navegacion, by Curbera, M. and Jimenez, M.). At Figure 2301 of the book is shown a vernier sextant which the text says is of the type “Guardiamarina” and constructed in Spain. One might be excused for thinking that “guardiamarina” means some form of marine force, but in fact the word correspnds to the English “midshipman” or “naval cadet” and we may conclude that it was intended as a low cost option for naval cadets. It was probably made by Empresa Nacional de Optica (National Optical Enterprise) or ENOSA, an enterprise dating from the period following the Spanish Civil War and the Second World War. Spain’s support for the wrong side in this latter conflict led her to be shunned by most European nations and the enterprise was apparently set up to manufacture optical goods of all types within Spain. A micrometer sextant, the Fregata, was also made but I have yet to see an example.

I was not previously aware that there were any Spanish sextant makers, at any rate in the twentieth century and feel that it deserves at least a footnote. Normally, I describe sextants in my collection or that pass through my hands as I restore or repair them for friends, but on this occasion I will describe one or two points of interest of the sextant based on viewing only an illustration. Not the least point of interest is that it was made in the second half of the twentieth century when manufacture of vernier sextants elsewhere had for all practical purposes ceased in favour of micrometer (or “drum”) sextants.

The illustration in Figure 1 is by courtesy of E.T.S. de Nautica y Maquinas de A Coruna, photo by Pablo Lopez Varela, and the original may be viewed at http://www.caosyciencia.com/visual/imagen.php?id_img=464 .  According to caosyciencia it dates from the 1960s.

Figure 1: Guardiamarina Vernier Sextant

Figure 1: Guardiamarina Vernier Sextant

To help readers whose first language may be Spanish and who wish to know the English equivalents, Figure 2 shows the same picture with labels giving the name of the parts in English.

Figure 2: English captions.

Figure 2: English captions.

The frame is I assume made of bronze and its pattern would not have been out of place in a late nineteenth century instrument. However, the size of the mirrors and shades, and the round horizon mirror would not have been out of place in the second half of the twentieth. Very unusual for a vernier sextant is the scale lighting system, while the unusual arm that carries the magnifier is provided with a diffuser screen for viewing the scales in daylight.  Chapter XXIII gives a careful explanation on page 182 of a vernier reading to 20 seconds which the authors say explicitly is of the same type as the Guardiamarina. Figure 3 gives a close-up view of the lower end of the index arm.

Figure 3: Close up view of lower end of index arm.

Figure 3: Close up view of lower end of index arm.

It is not possible to see how the release catch operates. On the face of it, the worm must be released from the rack in the plane of the frame, but it is not clear whether the  teeth of the rack are on the edge or the back of the limb. The angle of the tangent screw to the index arm seems to be against the worm bearing on the edge of the rack, while the arrangement of the release catch seems to suggest that it may do so. Perhaps the release catch operates a roller against an inclined plane, in so doing swinging the worm away from a rack machined on the back of the limb. Kelvin and Hughes used a similar arrangement in their Mark III Survey sextant, though to release the worm of a micrometer sextant (See “Sounding Sextants” on this site).

The telescope is of quite remarkable length. It is not possible to deduce its type. It may be a terrestrial telescope adapted to the sextant. At any rate, one can observe that the mounting bracket seems to be rather insubstantial. In a sextant reading to 20 seconds, there would be little point in a telescope exceeding a magnification of x3 or x4.

If any reader owns one of these sextants or has additional information, please do contact me with details and, if possible, photographs of the release catch mechanism.

C Plath Battery Handle Structure

5 12 2012

The preceding posts cover : “C Plath sextant lives again”; “C Plath Micrometer Sextant”; “A Damaged Rising Piece”, “SNO-T Mirror Bracket Repair”,  “A Worm Turns”, “The case of the broken screw”, and “Worm with wrong thread angle?”

A new friend recently sent me a battery handle from a C Plath sextant to reconstruct. The sextant had been stored under a sink which had had  a long-term leak and when the leak was eventually discovered, the lady of the house banished the sextant, in its case,  to the garage. As she had no key to the case, there the sextant languished for twenty years until my friend rescued it. The index mirror frame had corroded to a few scraps of aluminium alloy and the horizon mirror frame was nearly as bad. Corrosion had also affected the battery handle, which, fortunately, had no batteries in it, but the screw cap at the bottom had seized and both switch buttons lay free. He could press them in, but there was nothing to stop them dropping out. Some makers, and C Plath was one of the worst culprits, made their battery handles in such a way that it was impossible to deconstruct them if anything went wrong. In this case, corrosion had done the deconstruction, leaving behind a mystery as to how the parts should be re-assembled. Figure 1 shows the exterior of the handle. There are two switch buttons which operate independently to supply current to two sockets on top of the handle (Figure 2). A plug from one goes to the scale lighting system while the other was presumably to supply current to an artificial horizon attachment.

Switch 001

Figure 1: General arrangement of handle

Switch 003

Figure 2: Lighting sockets.

At the bottom of the handle is a brass bush or socket that is threaded for the battery cap. It is held in place by the threaded end of the lower pillar, which passess into a threaded cross hole and which also serves to conduct current from the negative pole of the battery to the frame of the instrument. A plastic liner reduces the interior diameter to suit two AA cells (Figure 3). The positive contact of the upper cell is held against the head of a large brass screw at the upper end of the interior of the  handle.

Switch 002

Figure 3: Lower end of handle deconstructed.

Figure 4 is a cross section drawing through the switch buttons.  The reduced diameter of the switch button passes through a phosphor bronze spring and then through a cross hole in the lower end of the socket. This serves both to capture the socket in place and to make electrical contact with it. A C-ring the other side of the socket sits in a groove and captures the button. When the button is pressed, the tapered end of the button makes contact with the large screw at the upper end of the interior of the handle. No doubt someone was congratulated for the elegance of this design, but of course, once the C-rings are in place the buttons and sockets cannot be withdrawn, as there is no means of accessing the C-rings to remove them.

Plath switch

Corrosion had done the job of removing the rings for me so my problem was how to  put everything together again. I began by removing the large screw to give the buttons some more  freedom of movement Having found a couple of small rings I did a trial fitting outside the handle and then greased a ring and sat it at the bottom of the hole, where the diameter of the hole reduces. Taking care not to forget the spring (which would have been disastrous) I then passed the reduced diameter of the button through the cross hole in the socket and entered the tapered end into the hole in the C-ring. A squeeze in a vice forced the ring up the taper and into the groove, where it will remain for ever. The other one received similar treatment and then the large screw contact was replaced.

At the lower end of the handle the problem was how to unseize the screw cap without causing unsightly damage to it by, for example, using a pipe wrench on it. One approach is to bore a hole in a scrap of wood to fit the cover. Then a radial split is sawn into the hole, the cover inserted into the hole and the wood squeezed in a vice to grip it. My solution was to make a bush to fit the cap from a scrap of thick-walled  aluminium tubing which I then split and squeezed in the jaws of a lathe chuck. I had no compunction about holding the brass socket with a pipe wrench to force the screw cap to yield and once it had yielded it was the work of moments to wire brush the corrosion away and remove any marks made by the jaws of the wrench with a smooth file.

The electrons now flow satisfactorily from the negative of the battery to the spring inside the cap and thence to the brass socket and via the lower pillar of the handle to the frame of the sextant.  From there they make their way through the lamp and the return wire to the plug which is inserted into one of the sockets. When the correspondng button is pressed, its end contacts the large screw and the current of electrons flows through the socket and the button return spring to the button and thence to the large screw, completing the circuit by entering the positive pole of the battery.