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).
Specifications for an operating circuit and switch settings under hot and cold conditions.
| Identifier | ExFiles\Box 59\1\ Scan320 | |
| Date | 30th April 1928 guessed | |
| -2- Contd. The operating circuit resistance comprises 1.45 ohms in the copper coil (at 180C) and 4.47 ohms in the Eureka. Total 5.92 Ohms, giving a ratio of Eureka to copper resistance of 3.1 as above stated. The resistance of the series field winding which is on the top of the Eureka portion of the main operating winding on what we call the "pivot bobbin" is of 5.16 ohms in a single layer of roughly 20 turns. It is the sudden impulse of field current passing through this winding which enables the points to separate definitely and thus prevents any tendency to vibrator regulator action. This is the feature of the auto charge switch which makes it possible to put in such a high resistance in the field that the dynamo is cutout except at very high speeds. The setting of the switch as tested by us previous to despatch is :- Cold Condition. Points separate when terminal voltage across main operating coil is 8.3. They close again when this voltage has dropped to 6.0. Hot Condition. The corresponding figures are 8.5 volts and 6.1 volts. Attached please find a copy of some notes relating to the fixing and adjustment of the switch. N.B. Hot condition referred to is that attained by heating up on a voltage of 8.5 continuously applied, the points meanwhile being wedged in contact. This setting is estimated to be about correct for operation on the car. EFC. | ||