Why 16 & 19 Jewels on English Watches but few with 17?

A 16 jewel movement by Errington of Coventry.

Several quality English makers made 15, 16, 19 & 20 jewelled watches but rarely, if ever,  used 17 or 21. The first reason to spring to mind might be cost, there may be some truth in that but I think the real reasons are the very conservative nature of most English watch makers and alternative and maker preferred methods of adding jewels.

Jewels are used because they are hard wearing and smooth so the pivot hole does not go out of round and friction is reduced, if only some wheels are to be jewelled it is best to use them on wheels that are moving quickly and with gearing effects have the most impact on reducing the overall power requirement of the watch.

The basic 7 jewels on a lever watch are 2 hole jewels + 2 cap (or end) jewels (to reduce end float and to keep the shoulders of the pivot off of the jewel to reduce friction) for the balance, an impulse jewel on the roller and 2 on the lever pallet. After this the next pair of jewels would normally be put on the escape wheel. After that there is some disagreement as to where they should go, the options being:

1. On the lever.
2. On the seconds wheel.
3. As cap jewels on the escape.

An 11 Jewel movement by Rotherham
with hole and cap jewels on the
escape wheel.

Some makers go for option 1 although the lever does not go through 360 degrees which is why some go for option 2. Rotherham in particular will go for option 3 to further reduce friction and to increase stability at this critical point, this may also be a cheaper option.

After 11, additional jewels would normally be added in pairs moving towards the centre wheel until you get to 15 (or 19 for a Rotherham with cap jewels on the escape and lever).

At this point it is worth mentioning that the marketing men sometimes prevail over the watch makers and jewels were added just on the top plate where they can be seen and not on the face plate. This is waste of jewels as those on the slower turning wheels contribute little and the benefit of the jewels that are there is reduced. I see this quite frequently on Swiss watches for the English market and occasionally on American watches.

English watchmakers did not like putting jewels on the centre wheel, this is probably a legacy of very strong main springs in big movements and a concern that the pressure would break the jewel. When pressed a lot of makers, notably Errington, would put a centre wheel jewel just on the top plate where it would be seen and not on the face plate where the maximum pressure from the barrel would normally be.

A rare 20 jewel watch by
Rotherham of Coventry.

Rotherham and many of the better Coventry makers would go to 19 by putting cap jewels on the lever and the escape, ignoring the centre wheel (they did not advertise the fact by engraving the movement as having 19 jewels so to the uninitiated they look like 15 jewel movements). Some excellent movements were made this way.  If pushed to show a jewel on the centre wheel they would then follow the practice of adding a single jewel to end up with a 20 jewel watch but these are rare birds.

In American makers would make a 19J watch by jewelling all wheels and then either adding cap jewels to the escape or jewels to the barrel, something not seen on English watches.