MAGNETIC METHODS 139 



sated by using a calibration curve which shows the correction for various 

 changes in temperature. This curve is usually plotted with the tempera- 

 ture correction in gammas as ordinate and the change in temperature 

 (t — to) in degrees Centigrade or Fahrenheit as abscissa. It is usually 

 convenient to assume some mean temperature for the area under inves- 

 tigation and apply the appropriate correction factor for each reading. 

 (As stated previously, thermometers are provided in the instrument so 

 that the temperature may be read at the time of each magnetic reading.) 



Certain instruments of recent design incorporate temperature compen- 

 sation, t and the effects of moderate temperature variations are usually 

 negligible with such equipment. In the temperature-compensated system, 

 two threaded rods are employed ; the rod on the north side of the axis is 

 made of aluminum (see Figure 49) and carries the temperature-adjust- 

 ment weight. The rod on the south side of the axis is made of invar steel 

 and carries the latitude-adjustment weight. The thermal coefficient of the 

 invar rod (carrying the latitude weight) is very small; hence, changes in 

 the position of the latitude-adjustment weight with variations in tempera- 

 ture are extremely slight. Furthermore, the slight change in position of 

 the latitude weight is largely compensated by the change in the position 

 of the center of gravity of the aluminum rod. 



Temperature changes greatly affect the scale readings when employing 

 auxiliary magnets. This is caused by the following factors: (a) change in 

 magnetic moment of the auxiliary magnet and (b) change in length of the 

 magnet support which is fastened to the base of the turntable. For this 

 reason, accurate measurements in an area must never be attempted with 

 auxiliary magnets. Oftentimes, auxiliary magnets are employed to bring 

 the scale of the instrument within view. This procedure is permissible 

 where large magnetic anomalies are to be measured. In regions of small 

 anomalies, the instrument must be opened and proper adjustment made 

 by rotating the latitude-adjustment weight until a suitable position of the 

 scale is observed. 



Temperature Compensation of Magnetic Systems. — The mechan- 

 ism of temperature compensation in the Askania magnetic systems is illus- 

 trated in Figure 54. As a magnet is heated it loses strength, with a resultant 

 decrease in its magnetic moment. For a magnetometer set up at a station, 

 at a given temperature a certain reading would be obtained. With an 

 increase in temperature the magnet system would become weaker and 

 therefore less strongly deflected by the magnetic force acting on it. This 

 would result in less tilt angle of the system and a smaller scale reading. 



As the temperature rises in a temperature-compensated system, how- 

 ever, the aluminum spindle carrying the weight Wi, which is on the north 

 pole side of the system, expands and shifts the center of gravity of the 



t Joyce, loc. cit., p. 42. 

 C. A. Heiland and W. E. Pugh, "Theory and Experiments Concerning a New Compensated 

 Magnetometer System," A.I.M.E. Geophysical Prospecting, Tech. Pub. 483, 1932. 



