The ELECTRIC GAUGE may be the thermostatic or magnetic type, as described previously. The sending unit (fig. 2-83) that is used varies, depending upon application.

1. The sending unit that is used with the thermostatic gauge consists of two bimetallic strips, each having a contact point. One bimetallic strip is heated electrically. The other strip bends to increase the tension of the contact points. The different positions of the bimetallic strip create the gauge readings.

2. The sending unit that is used with the magnetic gauge contains a device called a thermistor. A thermistor is an electronic device whose resistance decreases proportionally with an increase in temperature.

The MAGNETIC GAUGE contains a bourbon tube and operates by the same principles as the mechanical pressure gauge.

The INDICATOR LAMP (warning light) operates by the same principle as the indicator light previously discussed.

Figure 2-82.- Types of sending units for pressure gauges.

Figure 2-83.- Types of temperature gauge sending units.

SPEEDOMETER AND TACHOMETERS
Speedometers and tachometers in some form are used in virtually all types of self-propelled equipment. Speedometers are used to indicate vehicle speed in miles per hour (mph) or kilometers per hour (kph). In most cases, the speedometer also contains the odometer which keeps a record of the amount of mileage (in miles or kilometers depending on application) that a vehicle has accumulated. Some speedometers also contain a resetable trip odometer so those individual trips can be measured.

A tachometer is a device that is used to measure engine speed in revolutions per minute (rpm). The tachometer may also contain an engine-hour gauge which is installed on equipment that uses no odometer to keep a record of engine use. Speedometers and tachometers may be driven either mechanically, electrically, or electronically.