534 



EXPLORATION GEOPHYSICS 



be placed near the operator, who obtains the desired energizing current 

 flow by connecting an insulated clip to the proper battery terminal. The 

 energizing current is measured by a D.C. milliammeter having a range 

 of 0-10 milliamperes and a shunt switch with multiplying ratios of 5, 10, 

 and 100. A foot-controlled, sturdy contactor switch should be provided 

 for opening and closing the circuit. The current electrodes for energizing 

 the ground may be of the iron stake type. 



The potentials are measured by means of a D.C. potentiometer having 

 a range of 0-1000 millivolts. The potentiometer should be provided 

 with a double-acting, closed circuit switch or push-button control. The 

 galvanometer should be of the high resistance type, preferably 1000 ohms 

 resistance, and should have a current sensitivity of at least 0.005 milliam- 

 peres per scale division. A convenient zero adjustment to bring the 

 galvanometer needle to mechanical zero balance facilitates readings. The 

 potential difference existing between the two potential electrodes due 

 to natural earth potentials is neutralized or reduced to zero before each 

 reading by means of a neutraHzer or auxiliary potentiometer. This poten- 

 tiometer is energized by a 1^-volt dry cell. A reversing switch allows 

 the polarity of the impressed potential to be changed so as to oppose the 

 polarity of the natural ground potentials. A resistor i?i is employed 

 to reduce the potential impressed across the neutralizer potentiometer to 

 about 500 millivolts. Higher potentials than this make adjustment of the 

 neutralizer unduly critical. The potential electrodes are of the non- 

 polarizing type. 



In using this method considerable care is necessary to avoid errors 



introduced by : ( 1 ) the erratic 

 and unpredictable variations in 

 ground potentials in the poten- 

 tial measuring circuit, (2) the 

 rapid decline of the energizing 

 current due to polarization and 

 electrolysis phenomena adja- 

 cent the current electrodes, and 

 (3) failure to read the exact 

 value of energizing current 

 simultaneously with the ad- 

 justment of the potentiom- 

 eter for a zero balance on the 

 galvanometer. The effects of 

 the ground current variations 

 can usually be minimized by 



Fig. 325.— Simple millivolt-milliampere apparatus neutralizing the natural grOUnd 



for resistivity measurements, a, potentiometer battery • i j i • j* 



adjustment; b, high resistance galvanometer; c, ground potentials and thCU immecll- 



current neutralizer; d, potentiometer diaj; e, 0-10 mil- , « j- fVifk /^iirrpnt 



liampere meter; f, range switch for milliammeter giv- atCly reacting tnc CUrrcnX 



ing values to 2000 milliamperes; g, galvanometer irplnpQ -fnr <snmp «;ptfincr nf the 



shunt and potentiometer switch keys. VaiUCS lOr SOmC Seiting OI iric 



