MAGNETIC METHODS 133 



and in a vertical position with their axes parallel to the magnet system for calibration 

 of a horizontal component magnetometer. The scale reading of the magnetometer may 

 be brought to any desired value by using either the latitude adjustment screw or the 

 auxiliary magnet. If the auxiliary magnet is employed, caution must be taken to avoid 

 appreciable temperature changes during the calibration. 



When calibrating a vertical magnetometer it is carefully leveled and oriented with 

 the coils in position (Figure 56A). The scale reading and temperature are recorded 

 when no current is flowing through the coils. The switch is then closed and the scale 

 reading recorded with the value of current flowing through the coils. The direction of 

 current flow is then reversed and the scale reading again recorded with the value of 

 current. The circuit is opened and the zero or initial position of the scale checked. The 

 scale value of the magnetometer may now be calculated by the relationship : 



CI 2CI 



^ = (S.-s.) + (s; =^ '''^=s7^. ^55) 



2 



where E = scale value in gammas at the temperature of calibration, So = scale reading 

 for zero current, Si and ^'2 = scale reading with current in first one and then the 

 other direction. 



If the values of (So— Si) and (5*2— ^o) differ by more than a few per cent, the 

 condition of the knife-edges should be carefully checked. A series of observations 

 should be made with different values of current, when the north pole of the magnetom- 

 eter is pointing east and then west. Any variation in deflection will be shown by 

 irregularities in the calibration curves. 



In calibration of a horizontal component magnetometer, the instrument is oriented 

 in the magnetic meridian. As before, the values of initial and final scale readings, cur- 

 rent, and temperature are recorded. The values are substituted in the preceding formulas 

 and a series of tests conducted from which calibration curves may be drawn. 



One typical calibrating coil has a constant of 173.2 gammas per milliampere. With 

 the coil in place and no current flowing, a scale reading. So, is taken. With any given 

 value of current / flowing, a second reading is taken : Si. The scale value then would be : 



p_ 173.2' /milliamps. (K'^ ") 



00 — 01 



In practice, readings Si and Si are taken with the reversing switch in its two dif- 

 ferent positions, which changes the direction of the applied field. This procedure gives 

 a larger range of readings. The scale value when this is done is : 



P 2 » 173.2 • 1 milliamps. ('^Kh^ 



02 — o 1 



which is the general formula. A number of different values of current should be used, 

 to clearly establish the sensitivity-current relationship. 



Example : Determination of the scale value of Askania Vertical Intensity Magneto- 

 meter No. 90493, using calibrating coil. 



Scale readings Use Equation S5b above, with coil constant as given. Scale 



readings were at 1 milliampere of current. 



Si = 42.3 

 So = 27.2 



In the example 2 C = 346.4 



346 4.1 

 c„ — 1 1 9 E=z — -^— — = 11.4 gammas / scale division. 



30.4 



S,-Si = 30.4 



Slide rule accuracy is sufficient for these calculations. It is advisable to read the 

 So (or no-current value) in order to find if the differences Si — So and So — St are 

 approximately equal. If so, the instrument is functioning properly. 



