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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Analysis of the performance, issues, and potential solutions for running a set of lamps, focusing on voltage sensitivity and lifespan.

Identifier  ExFiles\Box 167\1\  img264
Date  15th June 1938 guessed
  
(1) The normal life of these lamps when run at their rated voltage of 3.5 volts, appears to be in the neighbourhood of 25 burning hours.
(2) They are very susceptible to small variation in loading, the life being reduced or increased considerably, according to whether the voltage is above or below the normal value.
(3) They are not made consistently accurate in the filaments, the inaccuracies producing loading variation which tend to shorten the life.
(4) Since they are susceptible to voltage variations which they inherently tend to produce themselves, they are unsuitable for working in series.
(5) The advantage claimed for this scheme is that the customer can obtain replacement lamps cheaply and from almost anywhere. Therein lies a weakness of the scheme, as obtaining lamps cheaply and from anywhere implies getting 3.5 volt lamps of various makes, wattages and degrees of filament accuracy which would only accentuate the trouble.
(6) A possible solution is to connect all six lamps in parallel each lamp having its own ballast resistance in series. If the resistances can be obtained cheaply, the advantage of the scheme may justify their extra bulk and expense.
(7) Provision must be made for running the lamps at not more than 3.4 volts. Even so there is danger where a lower wattage lamp is used of over-running.
(8) It will be necessary to ensure that the value of the ballast resistance lies within certain limits, and that the illumination with this arrangement on minimum system voltage is adequate.

Rm{William Robotham - Chief Engineer}/RC.{R. Childs}
  
  


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