318 Dr. Sumpner and Mr. Phillips on a 



or to similar causes of no present interest, the point being 

 that it was possible to adjust R to one part in 40,000, under 

 conditions of test which could easily have been rendered more 

 sensitive. The current used with the bridge could have been 

 quadrupled, and the strength of the field could have been in- 

 creased ten times, without injuring the apparatus, and with- 

 out altering the quantities under test. 



The method of fig. 2 was used to compare the capacity of 

 a paraffin paper condenser (about 0*9 m.f.) with the standard 

 mica condenser just referred to. The 4000 turn field-coil 

 w r as subjected to 20 volts on a 50 cycle circuit. From this 

 voltage was obtained, by means of a small transformer, a 

 cophase voltage of 2*5 volts suitable for the bridge conductors. 

 The resistances S were kept below 2000, and various tests 

 were made. It was always possible to adjust the balance to 

 1 part in 10,000, but the inconsistencies in the various tests 

 amounted to 2 parts in 1000 and are attributable to pheno- 

 mena (such as a partial conduction in the paper condenser) 

 affecting the exact formula for balance. 



An adjustable air-condenser, formed of a fixed and moving 

 set of plates like a multicellular voltmeter, was tested for 

 capacity against the standard mica condenser above referred 

 to. The maximum capacity was measured as 0*002)5 m.f., or 

 only 0*2 per cent, of that of the standard. . It was always 

 possible to measure the capacity in any position far more 

 accurately than the condenser could be adjusted to this posi- 

 tion. The method of fig. 2 was used, but higher voltages 

 were applied to the bridge than in the previous test. The 

 4000 turn field -coil was subjected to 30 volts, and this voltage 

 was also used for the condenser bridge, a resistance of 9000 

 ohms, made for the pressure circuit of a wattmeter and 

 suitable for high voltages, being used in association with the 

 air-condenser. The standard mica condenser was used as K x . 

 The associated resistance Sj was taken from an ordinary 

 resistance-box. It was set at various values up to 22 ohms 

 as a maximum. The 9000 ohm resistance was used for S 2 , 

 and the air-condenser K 2 was adjusted for each value of S L 

 till balance was obtained. The maximum voltage to which 

 Si was subjected in these tests was less than 0*1 volt. The 

 false zero method was employed for balance, the moving coil 

 voltage causing a deflexion of about 10 centimetres. 



It is to be noted that in all these bridge tests the phase 

 error of the electromagnet due to the resistance of the field 

 winding does not lead to any error, but merely causes a 

 negligible change of sensitiveness due to a shift of phase of 

 the moving coil current as compared with that of the field. 



