North American Mk1, and up to and including 1969 (other markets): The brush rubs on a brass cylinder attached to the column, but insulated from it. A wire comes from the brass slip-ring on the outside of the column up through the centre of the column to a bullet. This presses on a brass contact attached to the middle of the horn push. Operating the horn push connects the brass contact to a metal plate which secures the horn push to the wheel. The wheel and this metal plate are at earth potential, so when the brass contact touches it the horns sound. This series of photos are from John Minchin:
This ebay item shows the column gubbins more clearly. The box describes it as a slip-ring, although the slip-ring itself isn't shown, just the split plastic sleeve it slides over, and the wire that connects it up to the nipple in the end of the column. The original part number of this assembly is 7H 9603.
71-on: This is the brush (UK single stalk), fixed to the column outer, and rubbing on the brass ring on the back of the wheel as it is turned. Note that the original part number of this brush contact was 37H8102, which most places show as NLA. May 2014: However Brown & Gammons show this part as item 4, 37H 8051 for £9.54. Note also the brass slip-ring on the back of the hub, which is a replaceable item (BHA 5042) held on with two screws as shown in the next picture.
Wheel and horn push showing the various components. Note that the pencil is the wrong way round which was how I found it ...
... it should be fitted with the fixed contact uppermost so the insulating sleeve can prevent the contact touching the horn push body as it is fitted or if it is twisted once fitted. Update March 2010: Tim Schorn queried this saying he fitted a new one this way round and it very quickly broke, adamant I had it wrong. This has to be a faulty part, once fitted it only ever operates in compression. Not only is this logically the correct way round but Moss (Europe and US) and the Leyland Parts Catalogue all show it this way round. Tim says he paid $43(US) for this part, in which case he was robbed as they are only £8/$12 at Moss, even less elsewhere.
The component parts of the pencil (BHA 5041):
Twin-stalk models to the end of the 76 model year: There is a very small clearance between the indicator cancelling collar and the horn contact, Paul Bulley reports that his has broken in the past and been soldered back on, which has probably shortened it, and the horn sounds as the wheel is turned! Note this is with a Moto-Lita wheel so the contact has been 'adjusted' somewhat to suit.
The horn contact is riveted to an extension of the stalk switch clamp plate (via an insulator). Both stalks use the same clamp plate, the indicator switch (on the right) being clamped to the column first, the wiper switch then being screwed to the clamp plate. Although the switches themselves are available this clamp plate and horn contact doesn't seem to be from various sources like Moss, MGOC and Sussex Classics. However someone on the V8 Register BB said he got one from Mech-Spec in September 2008. Update September 2009: The remanufactured one by Moss USA below seems very similar to this.
Paul Bulley found this which seems to be the correct item for that period, from English Parts in the USA. Surprisingly cheap at under 9 Dollars - till you look at the postage which is 13 Dollars even to the USA! Also available from Leacy in the UK, said to be for 1970 to 74 models, but it differs in detail to my 73. The Parts Catalogue quotes the same part numbers for V8 and 2-stalk RHD cars for both switches and a single clip. LHD cars have different switches and no clips are specified. This could be an error i.e. the LHD part was missed off the list, or it comes as part of the LHD switches and not separately, or the clip that is listed for RHD cars should have been listed for LHD also. Only having two holes i.e. for one stalk it looks like it will only be suitable for Mk2 single-stalk cars, not twin-stalk, which would need an additional clamp bracket for the other switch.
Moss US shows a different version, which seems to be for the dual stalks as it has the offset holes for the two sets of fixing screws. It looks very similar to my V8 one above.
This is Michael Calandrino's 1974 (I think) US spec with a Mountney wheel. It shows a bracket with the offset holes, but horn contact is on the other side and also at a different angle to my (twin stalk) V8. It took me a while to realise that this is because LHD cars have the indicator switch on the left and the wiper/OD switch on the right!
The Moto-Lita wheel on my 75 V8.
And its connection arrangement. The wire coming out of the hub is 'permanently' attached to the back of the brass ring, but can be removed from the horn-push itself by undoing the nut securing the wire tag (to allow fitting of horn-pushes with different logos). This horn-push only has a single earthing/grounding spring, which means if you look carefully it sits very slightly offset in the hole in the hub. There is no arrangement to make the logo sit square in the wheel, it has to be twisted manually to suit. The actual switch contacts are concealed, and unlike the factory wheel (which has screws) this wheel can only be opened up by prising the spire-clips off the plastic pegs - something which must be done with great care if the pegs are not to break. All a bit cheap.
The pretty manky arrangement behind the Moto-Lita wheel. The slip-ring seems to be simply glued to the insulator which is glued to the back of the boss. This also clearly shows the single double-width clip which clamps to the column with the indicator switch, then the wiper switch is screwed to the clip.