A brief history of the A4 chime whistle - By Phillip Gooch
The type of chime fitted to A4's started life as a design from the US company 'Crosby', this was produced from 1877 until quite recently. BR copied the design at their Doncaster works but came across problems with the patent which, was owned by Crosby.
Following minor modifications to get around the patenting laws, many 3-tone chimes were produced for LNER A4's mainly but a few other engines had them fitted too. Perhaps the most famous of the A4's is 'Mallard'
The original Crosby chime is fitted on many preserved railway locos around the world, it is believed that there were/are a number on the New Zealand NZGR and the Australian New South Wales Railway although this has yet to be varified.
A basic steam whistle works not by blowing steam through the tube but by blowing it across the end of the tube. This is similar to the way in which you can play a note by blowing across the end of a bottle. This makes the air inside the tube resonate which gives the sound. The quality of this resonance and hence, the quality of the sound is affected by the type of material used which is traditionally brass.
The whistle works in the same way as any other steam whistle but, is equivalent to three different sized ones being played at once. This gives a 'fuller' sound - in effect, like playing a chord on a piano as opposed to a single note. The note played by each tube is determined its height (similar to the way in which you can change the note with the bottle by partially filling it with water) although the height of the arch at the base and the velocity of the steam flowing over this have a small affect as well. The approximate height of the Crosby tubes is 9 inches (with a further 3 inches of bell at the bottom).
The volume of the sound is determined by the overall diameter of the whistle, a Crosby chime is approximately 4 and a half inches in diameter. The diameter of the tubes could, theoretically be varied within the whistle so that some notes were emphasised more than others, giving a different sound to the chord.
The working pressure of the whistle is determined by two things, the height of the arch at the base of each tube and the flow rate of the steam across the opening. The flow rate of the steam can be varied by changing the size of the opening between the rim and the throat piece, a larger gap gives a slower rate which leads to the possibility of using a higher pressure. The flow rate could, of course, be changed using a flow rate valve but that wouldn't be very authentic would it !
As far as we know, we are the only ones mad enough to want to manufacture and fit a 3-tone chime of this size and type to an English road loco but, if you know differently or have any other information regarding the whistle please email me.