Sec. 3-1] 



MAGNETIC TRANSDUCERS 



187 



If a field gradient dH A /dx exists at the end A, an additional voltage 



e 2 will be induced. 



dH A d 2 



e 2 = nan ^ %; 2*/ sin (4tt/"«) 



In practice, e 2 is much smaller than e x , and since it is of twice the 

 frequency, it can be separated. 



If a very short, flat coil is oscillated longitudinally at A, it will 

 produce e 2 only, and there will be no signal due to H A . X 



H 



Drive 

 mechanism 



<?„=£>, +<? 2 



Fig. (3-1)8. Long search coil; one end A extending into a magnetic 

 field H A ; the magnetic field at the other end Z is practically zero. 



The method is suitable for measurements of flux densities in the 

 order of 50 to 100,000 gauss and for field gradients from 1,000 to 

 100,000 gauss/cm. For a coil of 1,150 turns, 0.1 cm radius, 3.5 cm 

 length, driven at a frequency of 1,000 cps with an amplitude of 0.001 

 cm, the sensitivities are 



A. 



rms 



pH A 



E 



2,rms 



= 0.46 x 10- 6 volt/gauss 



2.3 X 10- 10 volt/(gauss/cm) 



(x dH A jdx 



The long coil transducer is suited for applications in measuring 

 localized fields such as in magnetic lenses. 2 



Chapin 3 describes two oscillating magnetic-transducer systems 

 which are driven by piezoelectric crystals and which measure an 

 extraordinarily small area. The more sensitive system uses as a 

 pickup coil a single wire loop of 1 mm length and a total travel of 

 0.154 mm. It is driven by a bimorphic crystal at a frequency of 872 

 cps. Its sensitivity is 3 X 10~ 8 volt/gauss. The system is useful for 



1 J. F. Frazer et al., Rev. Sci. Instr., 26, 475 (1955). 



2 C. Fert and P. Gautier, Compt. rend. acad. sci., 233, 148 (1951). 



3 D. M. Chapin, Rev. Sci. Instr., 20, 945 (1949). 



