Oscillations in Coupled Circuits. 33 



The difference o£ potential between the plates of the 

 secondary condenser is therefore, 



2V 2 = + (T ^ T/2)cosg e . am (j -S) 



M| "° T ' ^Hr^J sin /± >a ^ 



~ (T 2 -T' 2 )cosS' * • Sm vT' V ( } 



(2) Experimental Arrangements. 



A detailed description of a short-period electrometer 

 designed by the author, suitable for the measurement of 

 rapidly-varying potentials, was given in a previous paper *, 

 and need not be repeated here. One or tw^o improvements in 

 the instrument may, however, be mentioned. The oscillating 

 strip has been made shorter, the distance between the sup- 

 porting glass rods being reduced to about 1*7 cm. This gives 

 the strip and mirror a shorter natural period of oscillation. 

 Instead of the oil-mixture described in the previous paper, 

 paraffin-oil is now used. This has suitable viscosity, gives 

 -very high insulation, and is colourless. In its present form 

 the instrument may be used for oscillations up to a frequency 

 of 1500, and the present suspension has lasted for nearly a 

 year with the strip under very considerable tension. 



The present experiments were made with two circuits for 

 •which the values of the product self-inductance x capacity 

 were not very different. The secondary coil was the secondary 

 of an induction-coil, of self-inductance, as measured by 

 Bayleigh's method, 70*15 x 10 9 cm., and resistance at the 

 time when the photographs were taken 14022 ohms. The 

 primary coil consisted of about 1200 turns of No. 14 copper 

 wire wound on a glass tube. Its resistance was 1*0378 ohms, 

 and self-inductance (by the same method) *004G19 x 10 9 cm. 

 This construction for the primary coil was adopted in order 

 to make it suitable for the " musical arc " method. 



Two leyden-jars were used as capacities in the secondarv 

 circuit, and two paraffin-paper condensers in the primary. 

 The values of the secondary capacities were determined in 

 the following manner. The terminals of the leyden-jar w r ere 

 connected to the secondary coil and to the electrometer. 

 Oscillations were set up in this circuit by sparking to the 

 terminals of the jar with an induction-coil whose primary 

 circuit was broken in each revolution of the rotating mirror 



* E. T. Jones, I. c. p. 238. 

 Phil. Mag. S. 6. Vol. 17. No. 97. Jan, 1909. D 



