This high current must of course be carried by the electrodes and the period just after ignition is particularly damaging. This is why frequent switching reduces lamp life.
As time progresses, the heat produced by this discharge begins to vaporise some of the mercury dose. The effect of this is to increase the voltage drop across the arc tube and more power is dissipated in the lamp so that its wattage increases. The increase in voltage is slow at first owing to the low power dissipation in the tube, but as more mercury is vaporised the power increases and the remainder of the mercury is evaporated more quickly, raising the lamp voltage simultaneously. As lamp wattage rises, so does the luminous flux. Similarly as lamp voltage rises, the current gradually falls to the normal operating value, relieving the load on the electrodes.
Once all of the mercury has been evaporated the voltage drop thereafter becomes practically constant. The discharge tube still continues to heat up, and as no more mercury is available to be evaporated, the quantity which has already been vaporised is then said to enter superheat conditions. The wattage continues to rise a little higher, further increasing the mercury pressure and improving the efficacy and colour properties. Final operating pressure is determined by the temperature of the coldest spot of the arc tube, usually just behind the electrodes. Because of this it is desirable to make the gap between the back of the electrodes and the end of the arc tube as short as possible. The precise gap here is limited by the strength of the glass, which will crack if it becomes too hot in the area of the glass-to-metal seal. A temperature of 360°C is the maximum value attainable in practice, and this corresponds to a mercury vapour pressure of precisely one atmosphere. At such high pressure the arc becomes constricted into a narrow cord of high brightness, as is clearly visible in Figure 28.
The whole running-up process takes about 9 minutes in the original designs of MA lamp, but is reduced for later models. If the mercury vapour pressure can be made to increase more rapidly then the lamp will run up faster, and in later lamps this was accomplished by applying a specular mirror of heat-reflecting platinum onto the ends of the arc tube just behind the electrodes to prevent heat losses here. The coating is always painted on in the form of platinum chloride which happens to be soluble in lavender oil, the resulting mixture being decomposed to a bright metal film by heating in air at around 500°C for 30 minutes. The effect is to reduce the run-up time to approximately 6 minutes in the case of 400W lamps, and 7 minutes for the 250W size. Faster run-up is advantageous not only for practical reasons, but because it takes the electrodes out of the high-load condition more rapidly and maximises their lifetime while reducing the rate of arc tube blackening. The location of the platinum coating is shown in Figure 29.
Figure 29 - Location of the platinum heat reflector on MA arc tubes is shown in grey
Illustrated in Figure 30 below is a graph showing how the voltage, current, wattage and luminous flux change while the MA type lamp is warming up to full power. If there is an interruption to the mains electricity supply and the arc extinguishes, it cannot be re-ignited straight away as the striking voltage of of mercury vapour rises very rapidly with increasing pressure. 5,000 to 10,000V is required to instantly re-strike a hot 400W MA lamp. Typically 10 minutes is required for the lamp to cool back down to a level at which the mains voltage will be sufficiently high to re-ignite it. There is then a further delay while the lamp runs back up to its full intensity again.
Figure 30 - Run-up characteristics of a 400W MA lamp with platinum end coatings |
