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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).
Description of the operation of a two-way 'Soft' and 'Loud' horn control system.

Identifier  ExFiles\Box 59\3\  Scan368
Date  13th November 1935
  
R.R. scheme
W.T. Horns

This system of horn control involves the use of a two-way push button switch, mounted on the steering wheel, and a double relay unit.

The knob of the push button switch is so constructed, that it can be rocked left and right about a centre point, making contact on either side and closing a corresponding circuit. The respective sides are marked 'Soft' and 'Loud'.

Reference to the diagram will shew, that when the knob is depressed on the 'Soft' side, the local circuit from the battery, through the fuse and relay magnet coil 'A' is closed. The current in this winding causes the contacts of the relay R.1 to close. These contacts in turn close the main circuit, so that a supply, for the horns, is obtained from the battery, via the .500 resistance; the resistance reducing the current to the horns and thus producing the soft note. Normally, however, this resistance is short circuited, by the contacts of the second relay R.2, which rest in the closed position. Simultaneously with the supply to the horns through R1, there is also a second supply through the winding 'C' of the magnet of R2 which causes the contacts of R2 to open, thus removing the short circuit.

When the knob is rocked to close the 'Loud' circuit, the local circuit, through coils 'A' and 'B' in series is closed. Coil 'B' is so wound as to oppose magnetically coil 'C'. Thus operation takes place as before, with the 'soft' circuit closed but now coil 'B' is in action which neutralises coil 'C', and leaves the contacts of R2 together. In this manner the full supply is available for the horns, and a loud note is produced.

It was found with this arrangement that in an emergency, the switch knob was usually pressed straight down, hard, closing both circuits. As will be seen, this short circuits the demagnetising coil 'B' rendering it inoperative, so that the result is equivalent to pressing on the 'Soft' side, at a time when a loud note would be desirable.

To overcome this defect, coil 'B' has been so arranged as to be supplied direct, and independently of coil 'A', and a second coil 'D' wound on relay R1 and connected in series with 'B', which takes the place of coil 'A' in closing the contacts of R1 when the 'Loud' circuit is closed.
  
  


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