16 Prof. A. Battelli and Mr. L. Maori on 



But we did not limit ourselves to this theoretical value in 

 calculating the results of our investigations, as it results 

 from experiments (not yet published) made for this purpose 

 in our Institute by Dr. Grragnani, that the above formula does 

 not hold for condensers of the dimensions used by us ; and 

 as, moreover, in this condenser each armature is formed by 

 two sheets of tinfoil separated by a plate of glass from 7 to 

 10 mm. in thickness, the departures from the theoretical 

 conditions this formula holds for are still greater. 



In addition, it is impossible to calculate with certainty 

 what influence the neighbourhood either of conductive masses 

 or of the walls and the floor may have on the effective 

 capacity. 



Moreover, though the plates as used with our experiments 

 are of mirror-glass and well worked, one cannot warrant their 

 being perfectly plane and accurately parallel among one 

 another. 



We therefore measured the capacity of our condenser, 

 either by comparison with a standard condenser or by deter- 

 mining experimentally the absolute value. 



Experimental Value. 



15. (1) By comparison with a Standard Condenser. — The 

 •standard, kindly lent by Prof. Roiti, hears the number 

 1099 of the firm Latimer Clark, Muirhead, & Co. (West- 

 minster), and is made up of plates of tinfoil separated by 

 sheets of mica. The value of the capacity as assigned by the 

 constructing firm is -1- microfar.; that found by Prof. Roiti* 

 by means of special determinations is 0*3359 microfar., very 

 nearly the same as the value 0*3336 found by Glazebrook f 

 for another standard forwarded by the same firm. 



It should be borne in mind, however, that in calculating 

 the absolute value of the capacity, Roiti adopted for the B.A. 

 unit of resistance the value 0*9883 legal ohms ; whereas in 

 the Chicago Congress of the year 1893 it was established that 

 1 B.A. should be equal to 0*98703 international ohms ; the 

 value as given by Roiti should hence be multiplied by 



A.QOQO 



-—-__- — 1*0013, in order to reduce it to the absolute units 

 0*98/03 ; 



now adopted. 



It thus becomes 



0*3359 x 1*0013 = 0*3363 microfarad. 



The comparison between this standard and our air-condenser 



* Nuovo Cimento [3 J xxi. p. 137 (1887). 

 f Phil. Mag. [5] xviii. p. 98 (1881). 



