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LV. On a New Method of determining Resistances. 

 By Thomas T. P. Bruce Warren*. 



THE following method of measuring resistances may be of 

 interest to electricians, as it supplies a means of ascertain- 

 ing resistances for a wide range without using a set of resistance- 

 coils, it being necessary to have only one resistance-coil of known 

 value. 



A condenser, charged from a constant battery, is discharged 

 through a galvanometer of known resistance, shunted with an 

 unknown resistance x ; a known resistance a is then added to 

 the shunt, and the discharge again taken. Assuming the de- 

 flections of the needle to be proportional to the quantities of cur- 

 rent, it is required to determine the value of the resistance x } and 

 also its shunt-power. 



Resistance of galvanometer . . . =5465 B. A. units. 



„ a (added to x) . . . = 300 „ 



Discharge, galvanometer shunted with x = 228 divisions. 



„ „ „ a+x = 347 ,, 



The multiplying-powers of the shunts will be inversely pro- 

 portional to the deflections ; consequently 



5465+^ . 5465+ (^ + 300) = . ^ . m 

 x x-\- 300 



or 



5465 + x /5465 + {x + 300)\ 



x~~ -\ ^+300 J [ '^> 



+ 300 



which gives for x 500 B.A. units, and 11*93 for its shunt-power. 

 The determination of x may be very much simplified if the gal- 

 vanometer be first shunted with the known resistance and after- 

 wards with the unknown resistance, or the two resistances con- 

 jointly. 



Let a (the known resistance) = 500 B.A. units. 



Discharge from condenser with 500 units shunt = 228 divisions. 



„ „ x „ = o47 „ 



Galvanometer resistance =5465 B.A. units. 



5465 + 500 

 500 



_/ 5465+^ 347 

 -\ x 7228' 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 40. No. 269. Dec. 1870. 2 G 



