210 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



where b is the length of the bar. The force can be balanced by placing 

 a weight of G grams on the other side of the scale. 



The method is applicable for strong fields (order of 10,000 oersteds) 

 „ and furnishes quite accurate results; the 



error is of the order of i of 1 per cent. 1 The 

 \ current is of the order of | amp, the length 



i K b about 0.5 cm. The sensitivity, i.e., the 

 change of weight on the scale per change 

 ,' of magnetic field strength, is of the order 



s 



of 0.1 mg/oersted. 



A similar arrangement is shown in Fi 



g- 



(3-1)37. A metal ribbon A (of silver or 



FiCx. (3-1)37. Wire-loop mag- , \ • i i i r -a 



netometer, alternate method, aluminum) is held between two springs 8. 



A current / passes through the ribbon. In 

 a homogeneous magnetic field H, perpendicular to the drawing 

 surface, a force will act on each element of the conductor A which is 

 proportional to the product IfiH dl. Under the influence of this 

 force the wire will deflect. The current / is varied until the wire just 

 makes contact with the electrode K. The magnetic field strength 

 can be found from 



»-? 



where G is a constant found by experiment. The instrument is useful 

 for field strengths above 10 oersteds. 2 



b. Thermal Methods. When a conductor is placed in an oscillating 

 magnetic field, eddy currents are induced in the conductor and heat 

 is generated. The resulting increase in temperature at a given fre- 

 quency is a measure of the magnetic field strength. 



The simplest embodiment of this principle is a mercury thermom- 

 eter. The mercury bulb is placed in an rf magnetic field, and the 

 temperature is read. The sensitivity of the arrangement can be modi- 

 fied by surrounding the thermometer with a metallic loop or with 

 ferromagnetic materials in which additional heat is produced by 

 hysteresis. Figure (3-1)38 shows calibration curves of a thermom- 

 eter with a bulb of 40 mm length and 5.5 mm diameter. In the 

 upper part of the curve, the sensitivity is relatively high, so that 

 field variations of 0.1 per cent can be measured. The disturbance 

 of the rf magnetic field by the presence of the metal may be 

 considerable. 



1 A. Farber, Ann. Physik, 9, 892 (1902). 



2 F. Schroter, Z. Instrumentenk., 44, 477 (1924). 



