OKLAHOMA ACADEMY OF SCIENCE 133 



is measured by the time of the test and the pressure in the chamber 

 above atmospheric. The pressure gauge is caHbrated with a pitot 

 tube. 



The calibration is checked by placing a known amount of uran- 

 ium oxide in the ionization cliamber. A current of air is passed over 

 it carrying the ions formed to the electrode. The rate of fall of 

 the leaf measures directly the radioactivity. 



A second check calibration is proposed. The capacity of the 

 electroscope may be determined in the following manner: A parallel 

 plate condenser is constructed with a guard ring. Its capacity may 

 be determined from its dimensions. One plate is movable and a 

 scale is fixed on it so that its distance from the other plate may be 

 accurately determined. The fixed plate of the condenser is at- 

 tached to the leaf system of the electroscope. The movable plate 

 is attached to the case of the electroscope. The guard ring is 

 around the fixed plate. The leaf system and guard ring are 

 grounded and the movable plate is connected to one side of a 

 battery of known E. M. F. The other side of the battery is 

 grounded 



The grounds to the leaf system and guard ring are removed. 

 Then the source of E .M. F. is removed from the movable plate 

 and this plate is grounded. This leaves the electroscope charged. 

 The movable plate is now moved away from the fixed plate and 

 since the capacity of the system is decreased its potential must in- 

 crease. The leaf of the electroscope rises and its position is read 

 in the telescope. 



The telescope scale is at right angles to the leaf support. Con- 

 sequentl}^ its readings are proportional to the tangent of the angle 

 that the leaf m.akes with its support. Now the potential of the elec- 

 troscope is approximately proportional to the tangent of this angle 

 so that positions read for the leaf are directly proportional to the 

 potential of the system. 



Consider the capacity C of the electroscope as constant. 



The potential V may be represented by V equals kd when d 

 is the position of the leaf read in the telescope and k is a constant 

 for a small variation in d. 



The condenser has a capacity A when the leaf reading is a. 

 Then we can say, 



Q equals ka(C plus A) 

 Change the capacity of the condenser to B so that (a minus b) is 

 small. 



Then Q equals kb(B plus C) 



and a(A plus C) equals b(B plus C) 



