||The first high pressure mercury lamp introduced in the USA was the H-3 model and it was essentially a copy of Philips' HP300, the original high pressure mercury development. Just like its European counterpart the H-3 was extremely short lived, and it was rapidly superseded by the H-4 model shown on this page.
One disadvantage of the H-3, like the Philips HP300, was its unusually high arc voltage of 230V. The lamp had been designed at this voltage so as to facilitate starting because at the time, quartz arc tubes contained only two electrodes. The third ignition electrode was located outside the arc tube, taking the form a nickel wire looped a few times around the arc tube body near the electrode at the cap end, and connected to the electrode at the opposite end. Control gear having a high open circuit voltage of around 400V was necessary for ignition with this technique, and since system efficacy is optimal when the arc voltage is roughly half that of the supply, the 230V arc voltage was established.
In the H-4 lamp a third auxiliary electrode is provided in the arc tube near the main electrode at the cap end, connected to the opposite electrode via a high resistance. The resistor takes the form of a wirewound coil encapsulated in a glazed ceramic body and can be seen above the stem press. The arc voltage of the H-4 was reduced to a more normal level of 130V which simplified the control gear circuitry. At the same time its power consumption was raised to 100W, vs the 85W of its predecessor. This lamp employs the same graded seals from quartz to tungsten rods as the earlier models, and the electrodes pre-date the modern oxide type, employing a small strip of thorium metal under the coil.