Short-Period Electrometer 



245 



(5) Measurement of the Capacities of the Condenser a. 



In order to test the accuracy of the periods of electrical 

 oscillation as determined from the photographs,, the following 

 experiments were undertaken for the measurement of the 

 various capacities and inductances in the circuit. In the 

 oscillation experiments four leyden-jars were used, connected 

 severally to the coil A, which in some cases had inside it a 

 second coil B, whose terminals were disconnected, connected 

 together or to a paraffin-paper condenser of rather large 

 capacity. The coil B consisted of 9 layers of No. II insulated 

 copper wire wound on a brass tube 58 cm. long and about 

 2*4 cm. in diameter, and had no iron core. 



The capacities of the leyden-jars were determined by 

 Maxwell's method. The method of charging and discharging 

 the condenser was that first used by Rosa *, in which two pieces 

 of platinum wire fastened with sealing-wax to the limbs of 

 the electrical tuning-fork dipped alternately into two mercury 

 cups connected to the points B, C of the bridge (fig. 6) 



The inner coating of the condenser was connected by fine 

 branched wires to the two platinum points. The galvano- 

 meter was a suspended-coil aperiodic instrument of about 

 500 ohms resistance, the battery 30 storage-cells. All 

 parts of the circuit were insulated, the insulation of the 

 condenser and its connexions being tested by a gold-leaf 

 electroscope. The resistances Q, S, P were taken from 

 resistance-boxes, and after each experiment were compared 

 with a standard box made by Nalder & Co. The value of Q 

 ranged from 10 ohms with the smallest condenser to 100 

 with the largest ; 8 was about 20,000 ohms, and P varied in 

 the different experiments from 10,000 to 20,000 ohms. The 



* E. B. Kosa, Phil. Mag. Oct. 1889. 



