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).
Investigation into distributor spark-over clearance distance and discharge voltages.
| Identifier | ExFiles\Box 142\3\ scan0328 | |
| Date | 1st September 1937 | |
| To By/RD. from Rm{William Robotham - Chief Engineer}/RC.{R. Childs} c. RM.{William Robotham - Chief Engineer} c. RG{Mr Rowledge}/CAP. MH{M. Huckerby} Rm{William Robotham - Chief Engineer}/RO.{C. C. Rose - Export Manager}1/1.9.37. 1204 Distributor Spark-Over Clearance Distance. In connection with the larger plug gaps, coming into use on the car, and the resulting necessity for the ignition coil to have a higher secondary voltage, the spark-over to earth clearance distance in the distributor head, has been investigated. Experiments carried out were based on the assumption that, in the event of a spark over, the leakage spark would take the shortest path from the distributor electrode, through the air, to the nearest earth point. Using a standard 25/30 distributor head, resting on a flat metal plate, the voltage required to discharge across the vertical distance between the face on one of the electrodes, and the earthed plate, was measured. The condition of heavy brush discharge, just prior to a spark over was taken, as it was considered to provide a margin of safety. Voltages at a supply frequency of 50 cycles, were used, as this afforded the most accurate method of measuring the voltage. Under these conditions it was found that with the head alone, the gap distance was .570", and the corresponding peak voltage for discharge, 13,760 volts. The gap was then increased by inserting various thicknesses of mica between the head and the plate, and the corresponding discharge voltage measured. The various gaps and their discharge voltages up to just over 15,000 volts are given below. | ||