34 Prof. E. Taylor Jones on Electrical 



as the spots were crossing the photographic plate. The 

 method of comparing the frequency, w, of the electrical oscil- 

 lations with that of the tuning-fork, making allowance for 

 difference of velocity of the electrometer and fork spots across 

 the plate, was fully described in the previous paper. The 

 self-inductance L 2 of the secondary coil being known, the 

 capacity was calculated from the formula C 2 = l/47r 2 L 2 ^ 2 , 

 which was shown in the previous paper to agree very closely 

 with the results obtained from the photographs. This method 

 has the advantage not only of giving the value of the secondary 

 capacity under approximately the required experimental con- 

 ditions, but also of giving at once the total capacity in the 

 secondary circuit, including leyden-jar, electrometer, and 

 coil. 



This method was also used for measuring the coupling- 

 coefficient M 2 /LiL 2 of the two coils. For this purpose one of 

 the leyden-jars was connected to the secondary coil, and the 

 oscillation curves were photographed with the primary coil 

 open and closed, the primary condenser being removed. The 

 ratio of the squares of the periods with the primary closed 

 and open is then 1 — M^LjLg. The mutual inductance of the 

 two coils was also deduced from these measurements, since 

 L : and L 2 were known. The values so obtained were 

 M 2 /L!L 2 = 0-1483, M = 0-2192x 10 9 cm. 



The method employed for the determination of the capacities 

 of the paraffin-paper condensers was as follows. The condenser 

 was connected directly to the terminals of the primary coil, whose 

 circuit also included a battery and an interrupter consisting 

 of a pointed zinc rod dipping under paraffin-oil into mercury. 

 The terminals of the secondary coil were connected (without 

 any condenser) to the electrometer. The mercury contact 

 was broken, by means of a plate attached to the axle of the 

 rotating mirror, as the spots were crossing the plate. Under 

 these circumstances the secondary circuit had only the small 

 capacity due to the electrometer and the coil, which was 

 determined by methods described in the previous paper *. 

 The frequency of the electrical oscillations in these circum- 

 stances is given by 



* L. c. p. 248. Co was here taken as the capacity of the electrometer 

 n— ° 1 



+ — 2 — 0+-- 0", where n is the number of sections in the coil, C the 



capacity of one section on the next, C" the capacity of the whole of the 

 secondary coil with the primary coil inside it. the value of C, was 

 •0000236 mfd. 



