The speedometer shows or indicates the speed at which the vehicle is running. It is usually made in combination with the odometer that registers the total distance traveled. The speedometer is driven by a gear mounted on the rear end of transmission put shaft, the motion being transmitted to the speedometer by means of a flexible cable.
The speed is indicated in kilometers per hour. The odometer registers the total distance traveled after the automobile comes off the assembly line. Some of the recent cars are equipped with two odometers, one being a small capacity type that can be reset to zero. This short distance odometer is usually called a trip meter.
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Types of speedometers
There are many types of speedometers, such as the centrifugal type, magnetic tape, and electric type. The magnetic type speedometer is most commonly used today.
(1) Centrifugal type
The centrifugal-type utilizes the centrifugal force of two ball weights. When the shaft turns, the balls move outward and raise up the toothed tube, which turns the pinion and pointer to indicate the speed.
(2) Magnetic type
This type utilizes the attractive force of a permanent magnet to indicate the speed. The construction is as shown in the figure. The rotating magnet is driven by the flexible wire. The induction plate is installed to the pointer shaft and turns together with the shaft. When the rotating magnet turns, a revolving magnetic field is produced, resulting in a torque acting on the induction plate. This torque causes the induction plate (pointer) to turn.
The torque becomes larger as the vehicle speed increases and the pointer corresponds by indicating higher peed. The indicating torque is balanced by a hairspring to enable the pointer to indicate steadily. When the vehicle stops, the hairspring returns the pointer to zero reading.
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