146 



EXPLORATION GEOPHYSICS 



The photographic plates used with the magnetometer shown in Figure 

 60 are 45 mm. by 60 mm. and are read by means of a graduated plate. 

 One millimeter of record equals 10 minutes time along the long axis of 

 the plate and approximately 10 gammas along the field strength scale. 

 (The latter is at right angles to the time axis or direction of movement 

 of the plate.) A record obtained with a continuous recording instrument 

 exhibits the minute variations which cannot be obtained with any type 

 of intermittent reading method. 



The Hilger and Watts record- 

 ing magnetometer utilizes either 

 the vertical or the horizontal com- 

 ponent magnetometers, which may 

 be clamped to the recording instru- 

 ment. A view of the instrument 

 set up in the field is shown in 

 Figure 61. 



Correction for Auxiliary 

 Magnets 



In regions where the anomalies 

 are large and precise measure- 

 ments are not required, auxiliary 

 magnets may be employed to ex- 

 tend the reading scale of the instru- 

 ment. This makes it unnecessary to 

 open the instrument and change 

 the settings of the latitude screws. 

 The preferred procedure for using 

 auxiliary magnets is as follows. 

 Observe the scale deflection at the 

 station where the end of the scale 

 has been almost reached, and record 

 the reading. Choose an auxiliary 

 magnet of suitable strength to 

 bring the scale reading to the de- 

 sired value. Record the new read- 

 ing with the auxiliary magnet; 

 record also the number or moment 

 of the magnet used, and the read- 

 ing on the extension rod. Then go back to one or two prior stations and 

 repeat the procedure. The average difference in the two sets of readings at 

 each of the stations is the auxiliary magnet correction in scale divisions for 

 that particular setting of the auxiliary magnet. This correction will then be 

 added or subtracted, as the case may be, for all readings employing the 

 auxiliary magnet. A similar procedure will be useful if it becomes necessary 



Fig. 61. — Ililger and Watts continuous re- 

 cording magnetometer. The recording apparatus 

 (A)_may be used in conjunction with either a 

 vertical (B) or a horizontal component mag- 

 netometer. The telescope of the magnetometer 

 is exchanged for a reflecting head (C) which 

 deflects the light beam into the recording appa- 

 ratus. The total swing of the light beam is 

 almost equal to the field of view of the mag- 

 netometer, with 1" of record width equal to 10 

 scale divisions of the magnetometer. Accuracy 

 of reading is about 0.1 scale division. (D) indi- 

 cates drive mechanism and (E) auxiliary magnet. 



