A New Electric Speedometer
Page 55
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ANEW electrical speedometer has been developed specially for commercial vehicles by S. Smith and Sons (Motor Accessories), Ltd., of Cricklewood, London, N.W.2, the design of which has been carefully arranged with a view to giving the utmost reliability and service. This news gains particular interest from the fact that the Ministry of Transport is at present considering the possibility of enforcing the provision of speedometers on goods vehicles, whilst they must be used on express carriages after December 31.
Au essential feature of the new Smith instrument is the wholly electrical connection between the unit driven from the transmission and the speed-recording mechanism on the dash, there being no flexible shaft or other mechanical
coupling. The first-named unit consists of a simple and robust electric generator, which can be driven from the main shaft of the gearbox (or in any other convenient manner), and which is not affected should lubricant find its way into the casing. The drive embodies a universal joint, thus making
allowances for any slight error in alignment.
The rotating part of the generator is simply a six-pole permanent magnet, which, in course of manufacture, undergoes an " ageing " process, said to make it practically immune from any decrease in effectiveness over a long period. Within the circular casing which surrounds this magnet is fitted a series of wrapped coils, in which the current is generated, and these coils are connected by twin wiring to the instrument on the dash.
Basically, the dashboard unit is a damped voltmeter, which records speed in virtue of the fact that the voltage produced by the generator bears a definite relationship to the rate at which the permanent magnet is rotated. A detail of interest is that the studs at the back of the instrument are arranged so as to be adaptable either for a bracket fitting or for a dashboard mounting.
On the occasion of a recent visit to the Cricklewood works we were able to see one of these new speedometers undergoing a calibration test, and were inpressed by the steadiness of the indicating needle. We were also able to examine the various parts, which appear to be extremely well made and nicely finished.