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Ignition of the arc was performed crudely but effectively by a most intriguing action which was adopted from the methods used to start a carbon arc. When energised, the full voltage would be applied across the length of the lamp - this being insufficient to prevent electrical breakdown of the internal atmosphere (essentially a hard vacuum plus the vapour pressure of mercury at room temperature). Of course, the required striking voltage can be reduced by decreasing the gap between the electrodes, and when one is fortunate enough to have a liquid electrode this is easily attainable! The entire tube would be tilted by hand, and with moderate skill a continuous electrically-conductive bead of mercury could be made to flow from the reservoir down to the iron anode. It would be allowed to touch the anode thus directly short-circuiting the lamp.
The lamp would then be tilted back to its original orientation, and a small arc struck between the anode and the bead of mercury as soon as contact was broken. It was then a simple matter to allow the mercury to run back down to its reservoir at the opposite end, thereby drawing the arc up to the full length of the discharge tube. It was a requirement to burn lamps at a slight angle with the mercury reservoir at the lowest point to ensure that the liquid electrode remained in its proper location thereafter. The complete manual ignition process is illustrated in Figure 13.
Figure 13 - Tilt-Start Method of Igniting the Cooper-Hewitt Discharge
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