Chemical and Physical Papers. 



57 



by increasing the total resistance of the entire circuit. A low- 

 voltage-pressure transformer, giving one or two volts, is best 

 adapted for supplying the alternating current. In this case only 

 one resistance and one reactance may be known, and therefore this 

 method can be employed to compare two capacities, two self- induc- 

 tances, or a self-inductance and a capacity. Figure 2 illustrates 

 the method of comparing a capacity with a self-inductance. 



Ri, Rj, and Rs are known adjustable resistances, wound nonin- 

 ductively. R4 is the resistance of the self-inductance, and is gen- 

 erally unknown. To find R4, use direct currents and balance the 

 bridge. Then disconnect the source of direct current and balance 

 the bridge with the alternating current, using the telephone re- 

 ceiver instead of the galvanometer. A balance is obtained when 

 the sound in the receiver is a minimum. If the capacity of the con- 

 denser is given and the self-inductance is unknown, it will be 

 easier to balance the bridge if two of the known resistances, such 

 as Ri and R3, constitute a slide-wire bridge, as their ratio can then 

 be changed quickly and continuously and the minimum of sound 

 can easily be determined. 



If the capacity of the condenser is unknown it will be better to 

 use a variable self-inductance at K who.se resistance is constant, 

 but if the self-inductance is constant use a slide- wire bridge for two 

 of the resistances. If Ri and R:i constitute the two segments of a 

 slide-wire bridge the condenser C should be connected in parallel 

 with R,. 



The following is an illustration of the use of the method in find- 

 ing the capacity of a condenser C (figure 2): 



Let Zi, Z2, Z3, and Z4 represent the impedances of the four arms 

 of the bridge. Since AD is a divided circuit, its impedance must 



