622 



EXPLORATION GEOPHYSICS 



Medium Frequency System 



Power for exciting the coil may be obtained from a suitable medium 

 frequency (100-500 cycles) alternator or from a conventional vacuum 

 tube oscillator. A rotating type alternator or a thyratron inverter tubet 

 may also be used. 



Fig. 389. — Alternator and its waterproof case, a, waterproof case; 

 b, 500-cycle alternator; c, driving gear; d, dry cells for field of alter- 

 nator; e, crank; f, waterproof lid covering control panel; g, cast 

 aluminum frame. 



For lower frequencies, that is, under about 25 cycles per second, it 

 is not practical to use either vacuum tube oscillators or rotating alter- 

 nators for the generation of a usable amount of power with portable 

 equipment. This is because of the large weight and bulk of the equip- 

 ment necessary to handle these low frequencies, due chiefly to the large 

 amount of iron required in the magnetic circuits. Where these extreme 

 low frequencies are desired, it is necessary to use a suitable commutator for 

 inverting a direct current supply. This type of equipment has been 

 described earlier. 



Figure 391 shows a medium frequency energizing coil consisting of 50 



t K Henney Electron Tubes in Industry (McGraw-Hill) ; also Industrial Electronics Reference 

 Book (John Wiley and Sons Co.), 1948. 



