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).
Test report comparing the effects of iron and copper tubes on spark gap length under insulated and earthed conditions.
| Identifier | ExFiles\Box 52\3\ Scan346 | |
| Date | 14th November 1919 | |
| Contd. -4- EFC {E. Fowler Clarke - Electrical Engineer} 1/T14.11.19. Iron tube. Length 5 ft. outside dia. .750", thickness .062". Copper Tube. Dimensions the same as iron tube. | Tube. | Length of gap for regular sparking. | |---|---|---| | | Tube insulated. | Tube earthed. | | None (cable straight). | .210" | -- | | Copper. | .205" | .035" | | Iron. | .210" | .035" | It will be seen from the above table that provided the tube is insulated, passing the high tension wire through it makes no appreciable difference to the length of spark obtained, its capacity in this case being a negligible quantity. If, however, the tube be earthed, the length of spark is considerably reduced owing to the capacity effect of the tube, also that it is reduced to the same extent for the copper tube as for the iron tube, showing that the effect is wholly electrostatic within the limits of measurement. A similar test was made using a Remy 6 volt 12 cylinder coil, with running current considerably reduced from normal by the insertion of resistance, from which exactly similar results were obtained as follows:- | Tube. | Length of gap for regular sparking. | |---|---|---| | | Tube insulated. | Tube earthed. | | None (cable straight). | .090" | -- | | Copper. | .090" | .035" | | Iron. | .090" | .035" | 1902 TYPOGRAPHICAL UNION LABEL DERBY | ||