134 THE UNIVERSITY OF OKLAHOMA 



or C equal (bB minus aA over (a minus b) 

 A and B may be computed from the dimensions of the variable con- 

 densers. 



Now let the potertia". initially en the system be V, i. e.. t''e poten- 

 tial of the battery used. Let the capacity of the condenser at this 

 position be D. Then we may write 



Q equals V(D plusC) 

 Also as above Q equals ka(A plus C) 



- and V(D plus C) equals ka(A plus C) 



k equals V(D plus C) over a(A plus C) 

 The variation of k can tlius be determined over the entire 

 ranie cf the scale and deflections can be read as potentials. This 

 gives the charge associated with any poition oi the leaf (since the 

 capacity of the electroscope is known). The rate of fa'.l of the leaf 

 can then be read as rate of loss of charge. This gives the ninza- 

 tion and consequently the radioactivity measured. 



Srmmary 



This elertroicope has sufficiently high sensitivity to measure the 

 expected radioactivity in s:ases. It will detect an activity unly 

 twenty tim:es as great as that present normally in the atmosphere 

 and will m.easure with considerable accuracy an activity but li".:lc 

 greater than this. 



Its construction is sim])le and all parts are easily accessible 

 for lepairs. It is sufiicient'y rugged and compact to be Ut^ed for 

 field work. 



Measurements m^ay be made with it m.ore rapidly than wi'ii 'ypes 

 of apparatus now in use. Rccaiibrations when necessary may be 

 easily and quickly made. 



XXX. SOURCES OF DIRECT CURRENT IN HIGH 



SCHOOL LABORATORIES 



B. C. Brruse 



Frcm Oklahoma City CoILgs, Oklahoma City. 



There ;!re a number or experiments bcth in the laboratory and 

 in the lecture-room that call for direct currents which must be 

 stronger than those which can be furni';hod by the ordinary prim- 

 ary battery. Since in many schools there are practically no other 

 sources of direct current than that jusc mentioned, it was thought 

 that it wotild not be amiss to call attention to som^e of the methods 

 of producing direct current and, a^ong with the different methods, 

 mention something of the expense involved in each. It should be 



