71(j Prof. Stroud and Mr. Gates on the Application of 



Up to the present we have carried out only the roughest 

 measurements of these very small capacities. For quanti- 

 tative experiments we propose to construct a small air- 

 condenser of variable capacity, so as to reproduce the 

 deflexion. 



Comparison of Capacities with Inductances of the Order of a 

 few millihenries and upwards. — The arrangement we adopt is 

 founded on that described by Prof. Anderson*. 



On the publication of Prof. Anderson's paper one of us saw 

 that the method could be improved by placing the high 

 resistance " r ''f in the battery-circuit instead of in the galva- 

 nometer-circuit. This resistance ^S^'' is frequently of the 

 order of 2000 or 3000 ohms ; and when joined in series with 

 an ordinary high-resistance D' Arson v^al in the galvanometer- 

 circuit, it reduces its sensitiveness very considerably ; whereas 

 when placed as shown in fig. 2, the effect of its resistance 

 can be annulled by increasing the potential of the battery, 

 the currents in the arms of the bridoe remainino- the same 

 as before. This modification of Anderson's method has been 

 in use at the Yorkshire College for ten years. The relation 

 between L and C may be proved to be 



The old method of working an experiment with a battery 

 is as follows: — The bridge is balanced for steady currents, 

 then the galvanometer-key is closed and the battery-key is 

 made or broken, whilst " r '^ is adjusted till there is no 

 induction-throw. The galvanometer we use is a 6(i00-ohm 

 Elliott pattern astatic instrument. The pressure is abour 

 1 7 volts, of which from 10 to 50 per cent, are applied to the 

 terminals d and e of the bridge. 



While getting the balance for steady currents it is con- 

 venient to join a carbon resistance-box in series with the 

 inductance. This carbon rheostat consists of 20 or 30 pieces 

 of ordinary battery-carbon which can be compressed by a 

 screw. It is preferable to put this adjustable resistance in 

 the arm eg and not in the arm ef as then no measurement of 

 this carbon resistance is required. As the inductances are 

 usually made of copper with a" considerable temperature- 

 coefficient, it is continually necessary to verify the steady 

 current-balance. If much difficulty is experienced in getting 

 the steady balance, it is frequently well to bank up the arms 

 P and R with non-inductive resistances. 



* Phil. Mag-, vol xxi. p. 329 (1891), or Electrician, vol. xxvii. p. 10. 

 t Fleming and Clinton, Phil. Mag. May 1903, Plate I. 



