186 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 3 



A different kind of oscillating coil is shown in Fig. (3-1)7. The 

 frame is placed in a magnetic field in the direction indicated and 

 oscillates through an angle 2a. The output from the coil is 



2tt 



Direction 

 of H 



Fig. (3-1)7. Search coil for rotary oscil 

 lation. 



E = — — anfcf./j,H 



V2 



where a is the area of the coil, n the number of turns, /the frequency 

 of oscillation in cycles per second, a the amplitude of rotation in 

 radians, and H the component of the field strength perpendicular to 

 both the axis of oscillation and the zero position of the coil axis. The 



output is not purely sinusoidal 

 but contains the second harmonic, 

 even for small angles of oscillation 

 (1 to 4°). Klemperer and Miller 1 

 have measured field strengths of 

 the order of magnitude of the 

 earth's field with an accuracy of 

 1 per cent; the sensitivity of 

 their instrument is 1.2 x 10~ 3 

 volt/oersted. 



Because the coil measures only 

 one component of the field, as de- 

 scribed above, two coils mounted 

 perpendicularly upon each other can be used to measure two com- 

 ponents of the field simultaneously without changing the position 

 of the apparatus. 2 



Figure (3-1)8 shows a long coil of many turns. One end is in a 

 region A where the field strength parallel to the coil axis is H A ; the 

 other end Z is well into a region of zero field strength, and the field 

 strength may fall off in any arbitrary fashion between A and Z. The 

 coil is oscillated in the direction of the axis with the amplitude d, 

 and the frequency/. The induced voltage arises from the changing 

 number of flux linkages and is described by Eq. (5c). It has the 

 instantaneous value 



*YICL 



^ = — pH A d2iTf cos (2tt/0 



where I is the length of the coil and n the total number of turns. 



1 O. Klemperer and H. Miller, J. Sci. Instr., 16, 121 (1939). 



2 J. Kuntziger and P. Luonn, Bull. Sci. AIM (Bel.), 18, 319 (1938); H. 

 Neumann, Arch. tech. Messen, V 391-5, September, 1939. 



