Chap. 10] 



ELECTRICAL METHODS 



647 



Bu77tr 



Fig. 10-13. Soil resis- 

 tivity bridge (adapted 

 from Davis). 



wire rheostat comprise the remaining bridge arms. 

 A telephone is used as null indicator. This in- 

 strument is primarily a low-resistance device and 

 may be used not only for soils but also for electro- 

 lytic solutions. The entire apparatus is portable 

 and is illustrated in the article referred to. 



The Zuschlag in- 

 strument for rock re- 

 sistivity determina- 

 tions (see Fig. 10-14) 

 is a regular A.C. (fre- 

 quency) bridge, and 

 provision is made to 

 determine both the re- 

 sistance and the ca- 

 pacitance of the speci- 

 men. Power of com- 



paratively low voltage is supplied from a beat- 

 frequency oscillator or buzzer of constant fre- 

 quency. If in this bridge pure resistances are 

 in three arms and an inductance and capacity 

 in the other, the phone will remain silent if 





Hi 

 Rz 



and if 



w = 



Fig. 10-14. Zuschlag imped- 

 ance bridge. 



(10-13a) 



Vi^CCo + Cl) ' 



where Cl is the distributed capacity, Rl the resistance and L the inductance 

 of the coil, Co the setting of the condenser, and Rq the setting of the resist- 

 ance in the other side of the bridge wnen balance is obtained. If a high 

 resistance specimen is now connected across the variable condenser, the 

 balance is disturbed and re-established by adjustment of the condenser 

 and resistance. Assuming that the new values to obtain balance are C^ 

 and R2 , the resistance of the specimen Rx and its capacity d are given by 



R.= 



i + [coCo(ie2-fio)]^ 



« Co \,Ri — Ro) 



and 



y (10-136) 



c. = 



1 -f- [o,Co(R2 - Ro)f 



If Rx is too low to obtain a balance, the specimen may be connected in 

 series with the induction coil and condenser. If C3 and R3 are the capaci- 



