INSTRUMENTS, OBSERVATIONAL PROCEDURE, AND CONSTANTS 



11 



Procedure for Observation of Ion Content 



1. Preliminary 



(a) Ascertain that the electrometer, batteries, and 

 electrical circuit are in working order. 



(b) Apply plate potentials to the electrometer and 

 start the air -flow motor ten or fifteen minutes be- 

 fore observations are to commence. 



(c) Select the range of scale divisions, say five or ten, 

 over which the electrometer shall discharge dur- 

 ing the observations, centering this range on the 

 scale division representing the earthed position of 

 the fiber; determine the potential in volts to which 

 the selected range corresponds. Record the range 

 as d and the voltage 5V on a suitable form (De- 

 partment form 101). 



2. Leak test or test of residual ion content with air flow 

 stopped 



(a) Stop air-flow motor and discharge central cylinder 

 and fiber. 



(b) For a selected period of say 100 seconds, observe 

 the fiber position at beginning and end of period. 



(c) Record on form 101 the time interval of the test 

 in seconds as Ati, and the scale readings of the 

 fiber at beginning and end as di- 



3. Ion content observations 



(a) Start air -flow motor; keep it running throughout 

 this part of the work. 



(b) Charge the central cylinder to a potential suffi- 

 cient to bring the fiber to a position slightly be- 

 yond the beginning of the selected scale range. 



(c) Note anemometer reading as fiber passes the be- 

 ginning of the selected range and again as it pass- 

 es the end, and record on form 101, under M. Re- 

 cord watch time Tl of first anemometer reading. 



(d) Repeat (a) to (c) to obtain as many sets as desired, 

 usually three or more. Record watch time T2 for 

 last anemometer reading. 



4. Leak test, etc. 



(a) Repeat test of section 2 above, recording the ob- 

 servations as At2 and 92 on the form. 



5. Repeat preliminary calibration 



(a) Redetermine the potential, V, for the selected 

 range 6 used during observations of ion content. 



Procedure for Computing Ion Content and Mobilitv 



1. From the anemometer readings observed under 3(c), 

 obtain AM in seconds and multiply this by the conver- 

 sion factor for the anemometer, 1.02 X 10^, to obtain 

 W, the number of cc per second of air passed through 

 the apparatus during the voltage drop 6V. For con- 

 venience in computation, obtain the reciprocal W"l, 

 in 10"'' units. Obtain W'^ for each set of observa- 

 tions under 3(d) also, and take the mean Wm'-^- 



2. From Ti and T2 and the corresponding anemometer 

 readings obtained under 3(c) and 3(d) determine the 

 total number of seconds during which the air was 

 drawn through the apparatus and the number of liters 

 drawn through. From this compute the time p for one 

 cc to pass through. (For the particular fan, motor, 

 and air -flow system used on cruise VH the value of p 

 was between 0.6 and 0.7 x lO'^ as the rate of airflow 

 varied between 1.7 and 1.4 liters per second.) 



3. From the value selected for Ati and At2 under 2(b) 

 and the value of 6 selected under 1(c), together with 

 the observed values of 9i and 62 obtained under 2(c) 

 and 4(a), compute values of t"! and t-2, in 10-3 units 



from the formula ti'l = ei/e(Ati) andt2-^ = 82/6 

 (At2). Take the mean tm"! of the two values and 

 multiply by the value of p just obtained, and record 

 as wm"^, in lO"'' units. 

 4. The formula for computing ion content, n+ or n., de- 

 pending on the sign of the potential applied to the 

 central cylinder, is 



n = C«V (w^-i . ^^-1) 



SOOe 



where C is the measured capacitance of the appara- 

 tus, and for ion counter 1 was found to be 24.2 cm. 

 Determinations of capacitance indicate the value to 

 be correct to within plus or minus two per cent. If 

 the value 24.2 cm is used, then 



C/300e = 16.8 x lo'' 

 and the working formula becomes 



n = 16.8 6V(Wm"^ - Wm'^) 



where the exponental terms lO'' and lO"'' cancel out. 

 5. Confusion may arise on occasion as to the sign of the 

 correction Wm'^. This may be clarified as follows. 



(a) If the travel of the fiber during the leak observa- 

 tion is in the same direction as during the obser- 

 vation of ion content, then 



n = 16.8 5V[Wni-l - (+Wm-1)] 



(b) If the travel during the leak observation is opposite 

 in direction to that during observation of ion con- 

 tent, then 



n = 16.8 5V[Wm-l - (-wm-^)] 



(c) These rules apply whether positive or negative ion 

 content is being measured. 



Ion counter 1 was installed near the conductivity ap- 

 paratus to facilitate simultaneous observations with both 

 types of apparatus. Form 101 was used to record both 

 ion content and conductivity as long as eye-reading in- 

 struments were used for both. Observations were so 

 adjusted that both types of measurement covered as 

 nearly as possible the same period of time, and the data 

 therefore can be regarded as taken simultaneously. 



From simultaneous data of ion content and conduc- 

 tivity the specific mobility of the ions can be determined 

 by means of the formula 



mobility, v, = A /300ne 



where v is expressed as cm per second per volt per cm. 

 Computed values of mobility were derived for all simul- 

 taneous sets of observations of ion content and conduc- 

 tivity and are tabulated later in this volume (pp. 66-112). 

 Aitken Nuclei Counters. --More than fifty years ago 

 John Aitken devised an instrument for counting the number 

 of particles in the atmosphere which act as centers or nu- 

 clei for condensation of moisture (2). A considerable body 

 of observational evidence has been gathered since that 

 time, showing that the number of these particles present 

 in any locality affects the electrical condition of the at- 

 mosphere there. Certainly when they are numerous, the 



