Electrodes

The electric current supplied to the lamp consists of a flow of electrons along the supply wires Electrons are very small particles which, together with other small particles, make up atoms The electrons continue to flow along the wires inside the lamp, and into the cathodes The cathode is usually a beehive shaped structure made from triple-coiled black tungsten wire To complete the electric circuit, the electrons must jump off one cathode and travel along the discharge tube towards the other electrode This requires significantly more energy than travelling along a wire, so the electrodes are coated with a special chemical calledemitter This reduces the amount of energy needed for the electrons to be emitted from (i.e. jump off) the cathodes In addition, the special beehive shape of the coil acts as a hollow cathode, further reducing lamp striking voltage The emitter is of the triple carbonate type, and consists of a mixture of barium, strontium and calcium carbonates suspended in an organic solvent The tungsten coils are dipped into the suspension and an electrophoretic process draws a precise amount of emitter into the coils The emitter must be thermally activated before it will work correctly, and this is done by electrically heating up the cathodes while on the discharge tube exhaust machine The lamp is evacuated and the carbonates are converted to oxides, producing carbon monoxide and carbon dioxide gases in the process:

            BaCO₃/ SrCO₃/ CaCO₃  ----->  BaO / SrO / CaO  + CO  + CO₂

It is important not to pump the evolved gases away immediately and to continueactivation a little while longer Some of the carbon dioxide then reacts with tungsten metal in the cathode to form tungstic oxide, and this in turn reacts with the metal oxides to produce the metal tungstates which forms a more durable emitter:

            3CO₂  +  W  ----->  WO₃  + CO

            3 (BaO / SrO / CaO)  +  WO₃  -----> Ba₃WO₆/ Sr₃WO₆/ Ca₃WO₆

Once activated, the gases are pumped away and the lamp is backfilled with a neon/argon mixture It is then lit while still on the exhaust machine, using special ballasts with a high open circuit voltage This cleans the glass and cathodes by ion bombardment and pumping is resumed Once activated the cathodes must not be exposed to air or moisture because this will poison them This could result in premature lamp failure

During lamp operation, particularly when starting, some emitter is thrown off the cathodes When there is no emitter left, the lamp will not strike so easily, and this is the end of lamp life Alternatively, if one electrode loses its emitter before the other, the lamp could rectify (it converts the alternating current supply to direct current) During rectification large currents flow which can damage the ballast’s windings It is for this reason that one of the outer lead wires in each lamp is thinner, designed to fuse if the lamp rectifies thus saving the ballast.

It is important to ensure that the correct amount of emitter is deposited on the cathodes If there is not enough then it will all be used up too soon and the lamp will have a short life If there is too much then it can flake off in large pieces and there is some evidence to suggest that this also reduces lamp life and leads to premature blackening of the tube ends The emitter coating should be over the tungsten coil only, and must not extend down over the clamp to the nickel lead-wires where these are used

2-Ply glass is also used to protect the area where the lead wires enter the discharge tube In this case the sodium resistant layer is on the outside of the glass A short length of this is fused onto each lead wire The area where the metal wire leaves the sleeving is not sodium resistant, so a magnesium oxide ceramic tube is attached over this point This reduces the possibility of sodium getting into the seal.

Philips lamps are made using a different technique which delivers a stronger seal No magnesia bead is necessary because the lead-wire is made of a single piece Nilo alloy, and a single sleeve of extruded pure borate glass is fused over these wires Being a single glass, rather than 2-ply type, the end of the seal is not weaker, thus no ceramic bead is required There is also some evidence to suggest that seal leaks can take place inside the dumet sealing wire itself, and this problem is completely eliminated with the Philips single-piece wire.

Figure 28 - Typical Electrode Assembly in a modern SOX lamp