Oscillatory Discharges. 17 



was made by discharging them successively through a ballisti 

 galvanometer, after bringing their armatures to potential- 

 differences whose ratio was known, and which were chosen 

 so as to have deflexions o£ the galvanometer-needle of the 

 same order o£ magnitude in both cases. 



The charging current was given by three Tudor accumu- 

 lators, whose circuit was kept permanently closed through a 

 thick argentan spiral, of a total resistance 51*14 B.A. 



To charge the standard condenser a deviation of the current 

 between one terminal A of the spiral and a point B distant 

 about -} G of the length of the spiral was taken ; to charge the 

 air-condenser the deviation was taken at the two terminals 

 of the spiral. 



The ballistic galvanometer was of the Du Bois and Rubens 

 type, the duration of a complete oscillation of its needle being 

 13 seconds. The charge of the condenser was kept on 1 second. 



From this comparison we obtained for our condenser the 

 value C = 0*016001 microf., this value agreeing very well 

 with the value = 0*015972 microf. obtained by substituting 

 for the resistances AB and AC of the rheostat above mentioned 

 two other far greater resistances, composed of two Edelmann 

 resistance-boxes of 530 and 9690 units respectively. 



The average of those two values gives for the capacity in 

 question 



C = 0*015987 microf., 



i e. 14388 electrostatic c.G.s. units. 



16. (2) Absolute Measurements of the Capacity. — In order 

 to have more reliable values for the capacity of our air- 

 condenser we decided to make, as above stated, determinations 

 in absolute measure also by the bridge method, as suggested 

 by J. J. Thomson* 36 ". 



The arrangement used by us is exactly the same as the one 

 previously adopted and described by Prof, Roitif. 



Let «, 6, d, g, and p denote the resistances of the three 

 branches of the bridge, the galvanometer, and the pile respec- 

 tively, and n the number of oscillations per second of the 

 tuning-fork ; the value of the capacity will be given by 



q_ a[{a + d+g )(a + b+p) —a 2 ] 



n\_(a + b +p) (a + d)— a{a + b) ] | [_ (a + d+g) {a + b) —a{a + cl)] 

 As an interrupter we made use of an electromagnetic 

 tuning-fork run by another tuning-fork in unison. These 

 tuning-forks, kindly lent by Prof. Roiti, are constructed in 



'"•• Phil. Trans, of tlie It. Sjc. part iii. p. 707 (1883). 

 t Nuouo Cunento, [:)] xxi. p. 137 (1887). 



Phil. Mag. S. 6. Vol, 5, No. 25. Jan. 1903. 



