Sec. 1-6] MECHANICAL INPUT TRANSDUCERS 133 



direction as a vector on the screen of a cathode-ray tube and is 

 accurate within 0.5 per cent for wind velocities of 50 mph. 



1-68. Transportatiox-of-charge Methods 



This method is illustrated in Fig. (1-6)13. Ions are produced at a 

 place P l in the flow duct; the ion cloud follows the air with great 





~~~ ~ 



1\6 



V///////////7. '//////. V///////////////77A 



Spark 

 generator 



Induction 

 electrode 



V///////////7?. 



R T ° 



amplifier 



Pos. charge 



A 



Fig. (1-6)13. Flow-velocity transducer system based upon the trans- 

 portation-of-charge method. 



fidelity without influencing the flow pattern. The presence of ions is 

 detected at a place P 2 downstream at a time At later. The flow 

 velocity can be determined from 



v = 



d 



At 



where d is the distance between P 1 and P 2 . 



The method measures the average stream velocity of the moving- 

 ion volume element. It cannot give information about the localized 

 or instantaneous velocity, and it cannot measure the total average 

 velocity. 



The ions are mostly produced within the gas stream by a high- 

 voltage spark discharge between two electrodes, the voltage being 

 generated either by an induction coil or by a pulse-modulated rf 

 source. The latter method is preferable; it permits the control of the 



