SPEEDOMETERS, instruments for measuring linear speeds —more particularly the speeds of road vehicles—and graduated in miles (or kilometres) per hour. They are driven either from one of the road wheels, or from the transmission, through a flexible shaft and a gear mechanism. Account is taken of the diameter of the road wheel and of the gear ratio between the wheel and the speedometer shaft. Speedometers carry a so-called odometer, which indicates distance.
A second principle very extensively used is that of magnetic drag. A cup of sheet aluminium is mounted on a spindle and held in the "zero" position by a spiral spring. A permanent magnet whose lines of force pass through the wall of the cup is rotated inside the latter at a speed proportional to that to be measured. The rotation of the magnet induces currents in the aluminium cup and the reaction between these currents and the magnetism of the magnet produces a drag on the cup, which causes it to turn around its axis against the resistance of the spring, and in proportion to the speed of the magnet. A scale printed on the outside of the cup shows through an opening in the face of the instrument, and the scale reading in line with a mark on the face indicates the speed.
A third principle made use of is based on the fact that speed is the quotient of distance by time. Instruments based on this principle comprise an odometer and a clockwork, measuring dis tance and time respectively, and they effect the operation of division mechanically.
A further principle is that of the magneto and voltmeter. A magneto generator has the characteristic that the voltage gen erated by it varies with the speed at which its armature is driven. Therefore, if a magneto is placed in driving connection with a road wheel and its terminals are connected to a voltmeter whose scale, instead of in volts, is graduated in miles per hour, the com bination will serve as a speedometer. This type is particularly adapted for use where it.is desirable to take readings at a long distance from the road wheel as the only connection is by wires. (P. M. H.) SPEEDWELL, in botany the name applied to the 17 Brit . _ ish species of the genus Veronica (family Scrophulariaceae) ; the British forms are herbs with bright blue flowers. The genus Veronica contains about 25o spe cies, temperate, alpine and arctic, many of which are shrubs or trees with handsome spikes of flowers, and are often cultivated The woody forms are especially char acteristic of the mountains of New Zealand. Some 15 species occur in North America, includ ing several that have been natu ralized from the Old World. The culver-root (Veronica virginica), found across the continent, was formerly much used in medicine.