The Nautical Sextant

This post was preceded by one on overhaul of the earlier Tamaya switches.

Early Tamaya sextant scale  lighting systems used a switch that was obvious in action and simple to overhaul. Later ones used a fully moulded handle that had no obvious way of accessing the switch contacts, an essential requirement of an electrical system working in a salt water environment. I covered the older Tamaya switch in the previous post in this category and have also covered the even more mysterious structures of a couple of C Plath switches in the “Interesting Overhaul Problems” category. In response to a recent enquiry, I now give an account of later Tamaya systems.

If we begin with the battery handle and remove the screw cap from the bottom we are faced with a deep dark hole, but if the light is right, one can see a spring at the bottom, secured to something with a plated brass screw. The end of the spring is supposed to receive the negative pole of the battery and observant owners will have noticed that there is a “+” sign engraved into the inside of the screw cap. Undoing the screw at the bottom of the hole allows the switch button with attached parts to be removed and when this is deconstructed one can see its structure, as shown in Figure 1.

Figure 1 : Structure of switch exploded.

Note the  contact screw that passes through a solder tag and a couple of washers into the sloping face at the upper end of the handle on the right. Figure 2 shows the parts assembled outside the handle. When the switch button is depressed, the end of the shouldered screw that passes through a bush and spring into the plastic button, moves to the right and makes contact with the end of the contact screw. We can now trace the current as it passes from the negative pole of the battery, through the spring and screw at the bottom of the hole, into the bush and switch contact and then into the contact screw on the side of the handle.

Figure 2 : Switch parts assembled outside handle.

The path of the wire that is attached to the solder tag in a Jupiter sextant is shown in Figure 3 . It is wrapped in a clove hitch around the upper leg of the handle to secure it and then passes through a hole in the index arm and  under a rectangular cover on the front of the arm to the bulb.  The path is similar in the Spica sextant but the bulb arrangement is different.

Figure 3 : Path of wire.

Figure 4 shows how the wire makes contact with the bulb. The wire is soldered to a thin bush and when the bulb is screwed into the holder, its lower contact makes contact with this bush. The lower contact is insulated from the threaded body of the bulb and the latter makes contact with the chassis of the sextant. The lower leg of the sextant is screwed into the side of the large brass bush into which the lower battery cap is screwed, so we can now follow the current from the side screw on top of the handle, through the wire to one contact of the bulb, through the bulb and its body, into the frame of the sextant and back to the positive pole of the battery.

Figure 4 : How wire contacts bulb.

Figure 5 shows the bulb in place in a Jupiter-type sextant and Figure 6 shows its position in a Tamaya Spica sextant, which employs a different shaped light guide to illuminate the scales.

Figure 5 : Bulb in Jupiter and simllar sextants.

Figure 6 : Position of bulb in Spica sextant.

If, like me, you love fine instruments and want to know more about the structure of the nautical sextant, may I suggest you buy my book “The Nautical Sextant”, available via Amazon, direct from the joint publishers, Paracay and Celestaire and from me, or through any good bookseller. You may also be interested in my latest book “The Mariner’s Chronometer” You will find in it a detailed illustrated account of the structure and function of the the marine chronometer with instructions on how to overhaul one with relative safety. There are also chapters on rating and transporting these delicate instruments and the book finishes with an historical chapter from a structural rather than social viewpoint and two appendixes for the advanced worker. It is available direct from Amazon and through large booksellers.

Most of the early Tamaya sextants and clones had the same design of battery switch, but I haven’t seen a modern one with two AA batteries for some time. I would expect Tamaya, in common with other makers, to have stuck with a simple design that was easy to overhaul at sea. This is what one of mine looks like:

Electricity flows from the positive battery contact clip through a short length of wire to the switch contact. Pressing the switch button causes the plunger, seen as a screw head, to bridge the gap to the other switch contact, allowing electricity to flow through a wire, poorly seen in the area of the switch, to the lamp. The circuit is completed via the frame to the negative battery contact ( and yes, I do know the electrons flow in the opposite direction).

Apart from bits breaking off, the battery going flat or the lamp burning out, what can possibly go wrong with such a simple arrangement? The short answer is “Anything that breaks the circuit”, but I’ll confine myself to the switch to begin with.

The switch contacts can get dirty or corroded and the same goes for the battery clips. The latter are easy to see and to get at to clean, so check those first. Even if they look fine, it does no harm to freshen them up by rubbing the contact areas with fine, e.g. 240 grit, emery paper. The switch contact areas are easy to get at. Simply remove the two screws that hold them to the handle and clean the undersides where they make contact with the plunger. While you’re at it, press the push-button in to prevent it from rotating and unscrew the plunger, taking care not to lose the spring that lies under the push-button. The head of the plunger, simply a shouldered screw, can then be freshened up too. Once the switch contacts have been removed, this is what you may see:

Note the bare copper wires that, given a chance, oxidise to a black, non-conducting colour.  You can freshen these up too, but a better course is to replace the short piece of wire with new, tinned wire (a piece of 15 A fuse wire will do) and to tin the end of the wire to the lamp with solder if you have a soldering iron and can use it. The other improvement you can make is to put the wires where you can see them, on top of the switch contacts rather than hidden underneath them, like this:

As a refinement, with a sharp knife cut a gasket from neoprene sheet to protect the contact area from salt water:

Assembly is the opposite of disassembly, except for placing the wires on top of rather than under the switch contacts. When replacing the switch plunger, take care not to overtighten as you screw it into the the push button. If you are too rough, you may split it and will then have to go shopping for some Araldite. The spring belongs underneath the button.

If replacing the battery and lamp and overhauling the switch doesn’t restore function, the fault most likely lies in the wire between the battery handle and the lamp, and it is most likely to break at the lamp itself. But that’s for another day.

You can read about other types of lighting systems as well as gain a knowledge of the detailed structure of your sextant(s) by buying my book The Naked Nautical Sextant and its Intimate Anatomy. Read what Trevor Kenchington has to say about it in “Inside the book”.