Chap. 8] MAGNETIC METHOD 355 



made periodically too negative to allow passage of electrons. The en- 

 trance of the electrons is detected by a direct current amplifier. By varia- 

 tion of the frequency of the oscillator, the interval between the emission 

 and return of the electrons can be gauged, since maximum reception is 

 obtained when electrons leave one window on one cycle and enter the other 

 on the next, or another wavepeak. If H is the intensity of the field, v the 

 velocity of the electrons, e/m their specific charge, and r their radius of 



V 2 . 



gyration, H = —. , or H = —j— ■ n, where n is the number of revolutions 



e/m-r e/m 



per second. Maximum plate current is obtained if the time of revolution 



is equal to the frequency / of opening or closing of the windows (or equal 



2 



to a multiple thereof). Hence, / = in, or H = . — i—-f. 



i-e/m 



A torsion balance for measuring magnetic and gravity gradients was 

 first developed by Eotvos.^" Berroth*^ perfected an instrument for the 

 determination of gradients of vertical intensity. In the magnetic torsion 

 balance, the hanging weight is replaced by a magnet suspended ver- 

 tically. Two runs have to be taken, one with north pole up, the other 

 with north pole down, since the magnetic axis of the suspended magnet 

 usually does not coincide with the vertical. Magnetic and gravity gradi- 

 ents must be measured separately, as the magnetic balance is also affected 

 by gravity forces. In Berroth's instrument one balance beam of a double 

 balance is used for the determination of the combined magnetic and 

 gravity effect, while the other measures the gravity effect alone. 



This procedure for measuring magnetic gradients is complicated and 

 requires an expensive instrument. Results are not in proportion to time 

 and money expended, considering that the vertical intensity gradient may 

 quite readily be obtained from vertical intensity maps and curves by 

 graphic differentiation. 



C. Instruments for Regional Magnetic Surveys 



Most of these instruments were developed by government and state 

 magnetic researcti institutions before the advent of the prospecting magne- 

 tometers. Their purpose is a complete determination of the earth's mag- 

 netic field, generally at fairly widely separated points. The most suitable 

 magnetic elements are declination, inclination, and horizontal intensity. 

 Instruments for the determination of these elements are called "magnetic 

 theodolites." They include, in addition to an astronomic attachment, 

 (1) an instrument for the determination of the magnetic declination 



*« Ann. Phys., 59, 354-400 (1896). 



« Zeit. Geophys., 9(6/8), 355-368 (1933). 



