Sec. 3 - 1 ] MA GN E TIC ' T RA SS DUC E i;s ] 8 1 



taking place during the time t to ^ and is independent of the rate of 

 change within this period. In practice, the magnetic field strength is 

 frequently changed from zero to the value H, so that AH = H, or 

 from the value —H to +//, so that AH = 2H. The integral form of 

 Eq. (16) gives « 



e dt = na AB (56) 



and, correspondingly, from Eq. (lc), 



= n A<I> (5c) 



The physical relationships expressed in Eqs. (1) and (5) form 

 the basis of all induction-type magnetoelectric transducers. They 

 differ only in the manner by which a change of flux is accomplished. 



The voltage e appearing at the output terminals can be measured 

 with any suitable meter which measures instantaneous values. The 

 voltage-time integral can be measured with any suitable electric 

 integrator. Ballistic galvanometers or fiuxmeters are frequently 

 used because of the sensitivities for small voltages generated by low 

 impedance sources. Many of the integrating methods require that 

 the integration time t x — t be short compared with the time constant 

 of the integrating device. For this reason the rise time or decay time 

 of the flux variation A® must be short compared with the time 

 constant of the integrator. 



a. Stationary Search Coils, d-c fields. If the input field can be 

 turned on or off or can be reversed or varied between defined limits, 

 a stationary coil can be used as a transducer and will furnish an 

 output according to Eq. (la) or (~xi). The method is not applicable 

 in the presence of ferromagnetic materials, which may cause a resid- 

 ual flux of unknown magnitude. There is no restriction in size, 

 shape, or number of turns of the coil, except that the coil should be 

 so small that the field is homogeneous over its area. A larger coil, 

 while still measuring correctly the total flux through its area, will 

 produce an output proportional to the average field strength or 

 flux density over the area of the coil. (For an exception, see the flux 

 ball, 3-1 lc). 



The constant of the coil, na, can be found with fair accurac}'" 

 from the coil dimensions. Kussmann 1 recommends the determi- 

 nation of na by counting the number of turns n and measuring the 

 length of the (unwound) wire. Higher accuracy may be obtained 



1 A. Kussman, in F. Kohlrauch, "Praktische Physik," H. Ebert and E. Justi 

 (eds.), 20th ed., p. 220, B. G. Teubner Verlagsgesellschaft m.b.H., Stuttgart, 

 1956. 



