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cavity changes, minute alterations in the shape of the membrane occur. As a result, electrical resistance in the embedded resistors changes. These changes in electrical resistance are interpreted by a circuit board and indicated on the variometer dial as climb or descent. 

Many electrical variometers provide audible tones, or beeps, that indicate the rate of climb or rate of descent of the glider. Audio variometers enhance safety of flight because they make it unnecessary for the glider pilot to look at the variometer to discern the rate of climb or rate of descent. Instead, the pilot can hear the rate of climb or rate of descent. This allows the pilot to minimize time spent looking at the flight instruments and maximize time spent looking outside for other air traffic. [Figure 4-25] 

Some variometers are equipped with a rotatable rim speed scale called a MacCready ring. This scale indicates the optimum airspeed to fly when traveling between thermals for maximum cross-country performance. During the glide between thermals, the index arrow is set at the rate of climb expected in the next thermal. On the speed ring, the 

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variometer needle points to the optimum speed to fly (STF) between thermals. If expected rate of climb is low, optimum interthermal cruise airspeed is relatively low. When expected lift is strong, however, optimum interthermal cruise airspeed is much faster. [Figure 4-26] A MacCready ring has single 

