138 EXPLORATION GEOPHYSICS 



cover with a hole in the center through which the telescope of the magneto- 

 meter protrudes. A small light on a rod inserted through another hole in 

 the cover illuminates the thermometers, which are read through a small 

 glass window in the cover. The mirror on the door in front of the thermo- 

 meters can be held open at the proper angle by a peg inside the heating box. 



While a temperature coefficient determination is being made, a second 

 magnetometer should be read from time to time to keep track of the daily 

 variation. A graph of scale reading versus temperature is drawn and the 

 coefficient determined therefrom. 



The temperature coefficient (Tc) is defined as-: the fraction of a scale 

 division which the scale moves for a temperature change of 1 C.° : 



A change of 0.9 S.D. for 4.0 C.° change in temperature shows a temperature coefficient 

 of 0.225 S.D./l C°.* If desired, the temperature coefficient may be expressed in 

 gammas per degree, knowing the sensitivity (or scale value) of the instrument. 



If a heating box is not available, the instrument can be placed in an oven (exercis- 

 ing due precaution), and carefully warmed, then set up and the readings, against fall 

 in temperature, recorded. It is also possible during hot weather to place an instrument 

 on a cake of ice to cool it and then observe the temperature change versus scale reading 

 as the temperature rises. The use of "dry ice" for cooling a magnetometer is twt 

 recommended as it might easily produce excessive chilling, with resultant strains, of 

 the metal, quartz and glass parts of the instrument. 



Heating elements are sometimes made to the same dimensions as the plastic plug 

 which screws into the bottom of the instrument. They can be built to use direct current 

 from a small portable battery or from the battery of a car. In the latter case a long 

 lead wire is provided, by means of which the magnetometer can be set up far enough 

 from the car so that it will not affect the readings. 



Temperature Effects. — The three chief effects on a magnetic system 

 due to changes in temperature are: (a) changes in magnetic moment (de- 

 creasing moment with increasing temperatures) ; (b) unequal expansions 

 of the component parts of the moving system with a resultant change in 

 scale constant, variations in optical constants, and changes in gravitational 

 moment accompanied by changes in scale value; (c) thermal and elastic 

 lags of the component parts of the system. 



It is the chief function of the heavy cork insulation of the case of 

 the instrument to minimize temperature variations — particularly, rapid 

 temperature fluctuations. Slow changes in temperature may be compen- 



* It is seen that as the temperature rises the scale reading decreases. Some magnetic 

 systems are over-compensated, and as temperature rises readings likewise increase. 



