180 



ELECTRICAL MEASUREMENTS 



kind of motion detector is a variable resistor. Any movement of the 

 movable contact along the "slidewire" changes the resistance in the 

 circuit, and this change becomes the electrical signal. Figure 13-5 shows 

 several such devices which respond to linear motion, rotary motion, and 

 pressure. 



rW\AWS 



Diaphragm or piston 



~> 



-* — *- 



Pressure 



B 



A C B 



rAAAAAA^ rJ 



B 



Fig. 13-5. Mechanical transducers in which resistance varies in response 

 to hnear motion (left), rotary motion of shaft (center), or pressure (right). 



A mechanical transducer could also produce changes in inductance or 

 in capacitance. For example, the capacitance of a condenser depends, 

 among other things, on the distance between the two plates. The Beck- 

 man InfraRed Analyzer (Chapter 10) employs a variable capacitance 

 detector. Some record-player pickup arms use variable inductances. In 

 both inductance and capacitance transducers, a moderately high-fre- 

 quency "carrier" current is varied by the signal because both inductances 

 and capacitances have their greatest eflFect at higher frequencies. The 

 variations in "carrier" current then can be amplified. 



Certain crystalline materials, quartz for example, exhibit the "piezo- 

 electric" effect. If a voltage is impressed across a wafer of the crystal, the 

 crystal changes its shape. If an alternating current is used, the crystal 

 vibrates, vibrating most strongly when the frequency corresponds to the 

 natural period of the crystal. This effect is used most commonly in regu- 

 lating the frequencies in communications circuits. The reaction is revers- 

 ible, however, making it possible to use such crystals as transducers. 

 If the crystal is compressed or vibrated, it will generate an alternating 

 voltage. This signal alternates with a frequency corresponding to the 

 vibration of the crystal, so the device can be used to detect small motions. 



Probably the most spectacular of the mechanical transducers are the 

 "strain gages." These devices depend upon the fact that the resistance of 

 a wire changes a tiny amount M^hen the wire is stretched. The wire (or 

 a flat strip of a conducting metal) is arranged so that the pulling force 

 is exerted on it, and at the same time, the wire is part of a Wheatstone 



