|
The troublesome effort of having to manually tilt every lamp in an installation to ignite the discharge is easy to appreciate, and it was not long before automatic methods were devised to electro-magnetically or pneumatically tilt the lamps and start them without human intervention. Of course, such methods were still rather cumbersome, and eventually a more scientifically orientated ignition method was devised.
Self-striking lamps were provided with an electrically conductive coating on the outside upper surface of the glass bulb directly above the mercury reservoir, so that there was a vacuum space between this external 'electrode' and the mercury in the lower portion of the tube. The conductive coating was connected directly to the anode at the opposite end of the tube - the intention being to increase the electric field strength above the mercury pool. The external electrode usually took the form of a ring of conductive paint applied to the bulb, but sometimes a wire hoop in close proximity to the bulb was employed, the general arrangement being illustrated in Figure 14.
Figure 14 - The Cooper-Hewitt Lamp with External Starting Electrode
In parallel with the lamp was wired a miniature mercury tilt switch, essentially a tiny scaled-down Cooper-Hewitt lamp itself, known as the 'shifter'. The shifter would be physically located directly alongside the series-wired inductive ballast which controlled the discharge current of the main lamp, and orientated such that in its resting position its own mercury dose would directly short-circuit the main lamp. Then on energising the system, a high current would flow through the windings of the inductance coil, bypassing the lamp by means of the mercury shifter which was short-circuiting it, the flow of current generating a strong magnetic field around the windings of the coil. The shifter was mounted very lightly such that it would be influenced by this magnetic field, causing it to tilt and drain its mercury charge to one end, thus breaking the circuit. The magnetic field built up around the windings would then collapse to zero at an extremely rapid rate, thereby inducing a high voltage kick which was sufficient to ionise the space between the mercury cathode and the external anode in the main lamp. (Incidentally, no ionisation takes place in the mercury shifter tube because it is itself wired in series with another small resistance, such that the route of lowest resistance is not through the shifter tube, but across the main lamp and its external electrode).
The small arc occurring above the mercury pool in the main lamp was sufficient to generate a good deal of mercury vapour and ionise it, and almost instantaneously the arc would then jump from the small external anode to the main internal anode electrode at the opposite end of the tube (of course, this proved a route of lower resistance since the main anode is internal, whereas the small external anode ring is dielectrically separated by the glass bulb).
If the lamp struck successfully then the magnetic field around the windings of the inductance coil would be quickly restored and it was still sufficiently great to hold the mercury shifter in its open-circuit position. However if the lamp did not strike up on its first attempt, the shifter would fall back to its resting position and short-circuit the lamp again, this entire process repeating itself as many times as was necessary until the lamp was successfully lit. A typical wiring diagram for the whole automatic starting arrangement is illustrated in Figure 15.
Figure 15 - Typical Wiring Diagram for Automatic Lamp Starting
The system of employing the external electrode plus auxiliary mercury shifter proved to be extremely reliable. Oil-filled or quick-acting circuit breakers were often tried as an alternative to the mercury shifter but proved temperamental and would not work all of the time. It is interesting to note that a higher induced voltage will be produced by the inductance coil for whichever device breaks the current flow through it the most rapidly - the magnitude of the high voltage kick of course depending on how rapidly the magnetic field around the windings collapses. Here the mercury shifter has an advantage, accounted for by the uniquely rapid deionisation of cold mercury vapour and the heat-dissipating properties of its volatile contact points. It was also observed that the colder the shifter, the more effective it would be. In hot weather the shifter would contain a greater pressure of mercury vapour itself, which resulted in a slower breaking of the circuit and less reliable lamp ignition (but still more reliable than any other form of rapid-acting circuit breaker).
|
