MAGNETIC METHODS 



101 



The principles embodied in instruments of this type open up many 

 interesting applications and will become of more commercial importance 

 as instrumental improvements are made. Instruments of this type prac- 

 tically eliminate the effects of diurnal and other magnetic variations for 

 which corrections must be made when absolute or relative magnetic 

 anomalies are observed. 



H(f^^ 



Magnetic Balance for Intensity Measurements. — Various instru- 

 ments have been proposed for accurate measurement of the earth's field. 

 The various modifications of the dy- 

 namic type appear to be the most suc- 

 cessful in elimination of fragile bear- 

 ings and other friction supports that 

 limit sensitivity and give difficulty in 

 field use. A majority of the dynamic 

 instruments utilize an electric means 

 for accurate neutralization of the 

 earth's field and for measuring the 

 current required for neutralization, t 



Knowing this current and the con- p^^ 31.-Balance for measurements of ver- 



t;tant<; of the instniment the streno-fh ^ical component of earth's magnetic field. 1, sup- 

 bldrUb Ol Uie lIlbLlUIUeUL, Uie bUeilf^UI p^^.^. j^ 1^^^^. g^ ^j^ror of optical system; 4, 



of the earth's field can be computed a"d 5, Hdmholtz coils; 6, energizing battery; 



'^ 7, rheostat; 8, potentiometer. (After Vacqmer, 



by use of fundamental formulas. u. s. Patent 2,151.627.) 



Vacquier Balance for Absolute Measurements of the Vertical Intensity t 

 The operating principle of this instrument consists in neutralizing the vertical 

 component of the earth's field by an opposing magnetic field created by an electrical 

 current. The apparatus (Figure 31) comprises a magnet which is conveniently though 

 not necessarily shaped as a bar and is supported for rotation about an axis very slightly 

 above its center of gravity by means of knife-edges. The latter are made of quartz 

 or other suitable material and rest on level quartz plates. (The plates are mounted 

 on a standard which has a base.) In use, the axes of the knife-edges are aligned 

 horizontally in a direction parallel to the magnetic meridian, so that only the vertical 

 component will affect the magnet. If the magnet were not magnetized, it would assume 

 a horizontal position (as shown) because of the small separation between the axis 

 of rotation and the center of gravity. Under the influence of the earth's field, however, 

 the magnet experiences a turning couple, the magnitude of which depends on the 

 vertical component of the earth's field at that station. The angular deflection of the 

 bar magnet is determined with the aid of a conventional optical system which consists 

 of a small plane mirror affixed to the magnet and a lamp, lens, scale, etc. 



The magnetic field for neutralizing the vertical intensity is obtained by means of 

 a Helmholtz coil arrangement. The electrical energizing and measuring circuit com- 

 prises the Helmholtz coils, a battery, a rheostat, and a potentiometer. The applied field 

 strength is altered by adjusting the resistance so as to bring the magnet to a standard 

 position. Measurements are made, therefore, on the null or balancing principle. 



t E. A. Johnson, "A Primary Standard for Measuring the Earth's Magnetic Vector," Jour. 

 Terrestrial Magnetism and Atmospheric Electricity, Mar. 1939, Vol. 44, No. 1, pp. 29-42. 



X Victor v. Vacquier, "Apparatus for and Method of Measuring the Terrestrial Magnetic 

 Field," U. S. Patent 2,151,627. Issued Mar. 21, 1939. 



