EVERYONE driving along the North Circular Road from the Edgware Road to Harrow must have noticed the handsome modern building occupied by British Jaegar Instruments, Ltd., and then looked back with increased interest at the ingenious instrument which is such an important feature of their dashboard. The delicate operations which go on inside this home of speed-recording make one wonder afresh at the skill which is lavished on the individual parts of the modern motor car. The mechanism of the Jaegar speedometer is carried between two brass plates, and making these was the first operation we saw. In one operation a power-operated punch cuts out a complicated outline in a strip of thick brass, makes holes, and replaces the punched-out piece in the strip, from which it comes out still a push-fit in its strip and undistorted by the pressing process. Holes of assorted sizes and depths are drilled simultaneously while the brass blanks are gripped in intricate jigs, while the multi-tappers, which reverse after cutting their threads, are

no less ingenious. The instrument cases are formed out of flat discs, and slots, screw-holes and other detail work scarcely less accurate is performed on them in a few seconds. Upstairs the shafts, pinions and other moving parts are manufactured on beautiful miniature lathes, holes down to i-millirnetre are placed in hard steel shafts with complete certainty, while we watched with interest a particularly ticklish operation in which a slot 5 mm. wide was being polished by a disc coated

with diamond powder and oil. Jigs again control every operation, while inspectors check the finished products. Dial-printing, though of course not such a delicate operation as those already described, is a fascinating process to watch. The figures and graduation required are deeply engraved in a steel plate, which is inked, and then the surplus ink is very carefully removed with a palette knife. An elastic gelatine

ball is then pressed into contact with the plate, and picks up the impression. The plate is then slid along and the dial takes its place, and a few pressures of the gelatine leaves a perfect impression. Now to describe the operating principle of the Jaegar speedometer, which differs from all other types on the market in that it is controlled by an escapement similar to that of a clock. Apart from the accurate readings which these instruments give, the needle is locked positively for short periods ‘of time, making them easy to read—even when negotiating

rough road at speed. A long article would be required fully to describe the mechanism, but the principle can be explained quite briefly. The speedometer is cable-driven in the usual way from the gearbox, or from

the engine when it is being used as a rev. counter, as the instruments differ simply in calibration. The drive is then taken through a chain of gears to an escapement, A—the normal balance-wheel and hairspring arrangement which regulates our watches and table-clocks. The final pinion of the train is not positively locked to its shaft, but drives it through

a friction washer, so that the shaft which carries the escapement wheel rotates at a constant speed irrespective of the speed of the driving pinion. As the diagram shows, there are two spindles C and D parallel to B. D is capable of being rocked and the small pinion E is alternately meshed and unmeshed with the large pinion loosely mounted on spindle C, by a rocking lever G actuated by a cam near the top of spindle 13. As the latter rotates at a steady rate E is meshed with F for a definite time, actually i-second, so that the wheel F will be turned by

E through an angle proportional to the speed of the driving cable and therefore to the road speed of the car. Wheel F has a small peg on its upper circumference which can butt against a coriesponding peg on wheel .H (the ” correcter “), and the correcter is rigidly connected to the shaft carrying the dial pointer I. Consequently, when E, start ing from rest, gives its first impulse to F, the peg on the latter wheel drags round H, and causes the pointer to show a reading. The moment E is unmeshed, however, a spring operated by a cam on spindle B

engages with teeth cut in the rim of H, locking it and the pointed, while a hairspring returns pinion F to zero position. The next impulse from the pinion E again rotates F a definite amount, proportional to the road-speed, where it is locked by a further ratchet spring operated by another cam on shaft B,

while H has been released and is either driven forward by the peg if there has been an increase since the last meshing of E and F, or flies back to it under the pressure of a hair-spring if the road-speed has become less.

In this way the needle clicks up or down each half-second according to the variations in speed and is locked in position meanwhile to give an easily-read indicator. The upper disc on shaft C serves further to steady the motion of the spindle and is subject to a continuous pressure from a leaf spring. The mileage-recording mechanism is much simpler but equally ingenious. Four or five discs, according to whether trip or seasonal mileage is being recorded, are assembled on a shaft, and the one giving the lowest figure of the set is driven by worm-gear and then through a pawl and pinion mechanism to give the final reduction, from the cable-shaft of the speedometer. At one point on its in ner rim a cam is formed and once per revolution this lifts up a leaf spring which has been holding its neighbour, ind when this is released the friction between the two discs is sufficient to turn the second one. The second one, however, has ten cams on its circumference and therefore only moves a tenth of a turn before being caught up and brought to rest. It can be appreciated that with one cam on the inner rim of the ” units” disc and ten on the “tens,” and one on the inner rim of the ” tens ” and ten on the “hundreds,” each disc in succession turns round at a tenth of the speed of its

neighbour, which is what we want. The friction of one disc against its neighbour is maintained by a coil spring at

one end, while they are returned to zero by a bevel-drive rod which first of all disengages the drive from the main shaft. After assembly the speedometers are run in on special machines for 48 hours or more and then tested for accuracy against a master instrument all the way up through the scale. If required these instruments can be made dead accurate at 60 m.p.h., and not more than 2 per cent. fast at 100 m.p.h., but many car manufacturers demand fast readings, and the instrument-maker must supply what is wanted. One meter intended for some small car was showing a steady 80 m.p.h. while the most truthful dials indicated a mere 65! Happily this childish custom is now disowned by most reputable sports car firms and the handsome dials of the Jaegar speedometers

and revolution-counters give readings as accurate as they are definite and easy to read.

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