Sec. 1-2] 



MECHANICAL INPUT TRANSDUCERS 



25 



mercury; the error is about 1 per cent of the output for maximum 

 rotation. The influence of temperature variation is very small, about 

 0.005 percent/ *'. 



Higher resistance values are obtained by winding the resistance 

 wire around an insulating core or mandrel. Figure (1-2)3 shows an 

 arrangement for the conversion of longitudinal displacements, Fig. 

 (1-2)4 shows the common form of the cylindrical potentiometer suit- 

 able for the conversion of limited angular displacements into electric 



Mechon_icoJ_ 



input 



II I I I I I I 1 1 I 





oB 



Fig. (1-2)3. Resistive transducer for 

 longitudinal displacements. 



Fig. (1-2)4. Potentiom- 

 eter for angular dis- 

 placements; cp < 360°. 



Fig. (1-2)5. Potentiom- 

 eter for angular dis- 

 placements. 



signals, and Fig. (1-2)5 the same potentiometer for continuous rota- 

 tion of the brush arm. 



The resistance value of such potentiometers ranges from several 

 ohms to several megohms, that of precision potentiometers for com- 

 puters generally does not exceed the order of 10 5 ohms. 



The resistance in such potentiometers increases in steps, rather 

 than continuously, when the contact moves from one turn of the 

 resistance wire to the next. This stepwise variation limits the reso- 

 lution of the potentiometer. The difficulty can be reduced, and the 

 resolution can be increased, by the use of very fine resistance wire, 

 permitting the increase of the number of turns of resistance wire 

 per unit length of the core and thus decreasing the size of the steps. 

 The resolution of circular potentiometers is frequently expressed by 

 100/?i in per cent, where n is the number of turns of resistance wire 



