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).
Experiments and comparisons of electrical coils, discussing insulation, performance, and testing standards.
| Identifier | ExFiles\Box 45\4\ Scan092 | |
| Date | 18th November 1922 | |
| R.R. 109 (250T) (SD676 19-7-17) MPuR/805 (4) EFCl/T18.11.22. Contd. (18) The experiment of more highly insulating the H.T. wire and terminal by means of giving the H.T. terminal insert a coating of rubber compound before moulding has now been tried on an otherwise similar coil, and has shown no material improvement. We therefore are still of the opinion that when the outer layers of the secondary are near to the case the effect, composed partly of leaks and partly of dielectric hysteresis loss exists. (19) Regarding the superiority of stabelite coils, this has been definite every time. We can say without hesitation that we have not had a coil in a case of bakelite or similar material which has been equal as regards this effect to the ordinary standard 40/50 or other coil in a stabelite case. (20) Regarding the obtaining of a figure of merit for a coil, to begin with it is extremely difficult to get a standardised spark gap for this relatively small sparking potential, which will break down consistently at the same potential. So much depends on the actual shape of conductors in the immediate vicinity, cleanliness of the points, impulse ratio, etc. etc. that it is really impossible to have a standard. The sphere gap which is used very successfully for extra high voltages would be too small at these smaller voltages to be manageable. Then again, a good deal depends on the exact form the spark at the L.T. points takes on each successive break, i.e. the damping effect is very irregular. We did attempt to do it with varying success as reported in the case of various magnetos we have tested, by using leakages of various values across the H.T. gap. As far as we can do it, we compare coils at ordinary atmospheric temperature with the standard ballast on 8 volts and 6.5 mm gap, at 700 R.P.M. of the distributor. The actual variation in the ballast resistance round about working values in order to compare coils gives results which are too erratic from which to form any systematic conclusions. By increasing the ballast resistance abnormally a little better consistency may be obtained. (21) We think it would be rather difficult to get an insulation resistance of the secondary circuit of 10 megohms at a temperature of 90°C. (22) The changes in the phenomenon of the touch of the fingers on the coil we think we can explain. When the coil is cold, one can just feel a slight knocking effect which, as you suggest, is probably due to the attraction and repulsion due to electrification. When the coil case Contd. | ||