102 



EXPLORATION GEOPHYSICS 



Fig. 32. — Schematic diagram of 

 induction variometer. (McNish, 

 Rev. Set. Ins., Sept. 1936, p. 338.) 



The auxiliary electrical circuit shown in the right 

 hand portion of Figure 31 provides a reversible mag- 

 netic field for reversing the direction of the induced 

 magnetism of the bar magnet. This field is used to 

 compensate the effect produced by a possible dis- 

 placement of the center of gravity of the bar magnet 

 due to warping, temperature effects, dust on the 

 knife-edges, etc. The balance obtained by adjusting 

 the current in the Helmholtz coils is the true balance 

 if the scale deflection remains constant on reversing 

 the induced magnetism of the bar magnet by means 

 of the auxiliary magnetic field. 



It is evident that the intensity of the applied field 

 can be determined absolutely provided the instru- 

 mental constants are known. 



Iron Induction Instruments. — The 



effects observed with iron-induction instru- 

 ments are produced by magnetization induced 

 in iron bars by the earth's field. The iron- 

 induction instruments described below are 

 used for determining the vertical component 

 of the earth's field and the magnetic meridian. 



Induction Variometer 



The design of a recent type of induction variometer employed by the 

 Carnegie Institution of Washington for vertical intensity measurements 

 is illustrated schematically in Figure 32.f Four staves milled from a per- 

 minvar bar serve as field pieces.* These field pieces are mounted with their 

 long axes vertical ; hence, the induced magnetization in the direction of 

 the long axes depends on the vertical component of the earth's field only. 

 Two smaller pieces, milled from the same bar, serve as armature pieces. 

 The latter are mounted symmetrically on two sides of a frame of brass 

 to which a mirror is attached, the assembly being suspended by a quartz 

 fibre. Variations of the magnetic fields associated with the induced mag- 

 netization of the field-pieces cause a rotation of the armature, and, as usual 

 with suspension instruments, this rotation is restrained by the torsion of 

 the suspension fibre. The angular deflection, which depends on the vertical 

 component of the earth's field, is measured with the aid of a telescope by 

 observing the deflection of a beam of light reflected from the mirror at- 



t A. G. McNish, "An Induction-Variometer to Measure Magnetic Anomalies," Rct. Set. Ins., 

 Vol. 7, 1936, pp. 336-338. 



* Perminvar, which is an alloy of 45 per cent nickel, 25 per cent cobalt and 30 

 per cent iron, shows an extremely small hysteresis loss. A comparison of the 

 magnetic properties of perminvar and other less highly magnetic alloys is given by 

 G. W. Elamn, "Magnetic Alloys of Iron, Nickel and Cobalt," Electrical Engineering, 

 Dec. 1, 1935, pp. 1292-1299. 



