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).
Letter from the American branch to the Derby branch regarding the pros and cons of 6-volt vs 12-volt bifocal lamp bulbs.
| Identifier | ExFiles\Box 61a\1\ scan0389 | |
| Date | 30th October 1928 | |
| ROLLS-ROYCE OF AMERICA, INC. SPRINGFIELD, MASS. 44477 OY2.R.10.30.28 October 30th, 1928. Mr. Robotham, Rolls Royce Ltd., Derby, England. Dear Mr. Robotham: RE BIFOCAL LAMP BULBS - SECTION 7460 Replying to yours Hs{Lord Ernest Hives - Chair}/Rm{William Robotham - Chief Engineer}10/LG17.10.28 I am in touch with the General Electric Corporation about 12 volt 2-filament bulbs and will write you shortly. Because of the greater length of filament and consequent difficulty of getting the filaments accurately at the focal point, there has been difficulty in getting a satisfactory 2-filament 12 volt bulb. This may be overcome now. If not will send you 6 volt bulbs which will call for "splitting" a 12 volt battery and running each bulb off half the battery. (This is one disadvantage of the 12 volt system.) An advantage of the 6 volt system which we have recently appreciated is the fact that there is still a large output of small 2 and 3 candlepower 12 volt bulbs for various uses, and that these in conjunction with a 6 volt lighting system give an excellent dim illumination for such things as instrument boards, "back-illuminated" instruments like the King-Seeley gauge etc. The successful use of bifocal bulbs in headlights entails certain details in the lamp construction which we are only beginning to appreciate fully. (1) The parabola should have a focal length not much greater than the S.A.E. standard of 1-1/4". Our present parabola has 1-3/8" focal length and works well enough, but 2" or upwards is quite useless as the shift of the light center is insufficient relative to the dimensions of the parabola and the light is not thrown down enough to do any good. -continued- | ||