X. ULTRASONIC VIBRATIONS 



317 



to sound energy. Since eddy currents are generated in a plane nor- 

 mal to the direction of the magnetic field and their magnitude is 

 proportional to the square of the width of the conducting metal in 

 this plane, it is common practice in magnetostriction devices to use 

 thin-walled tubes instead of rods and to slit these tubes in the direc- 

 tion of the magnetic field. The tube need be split only over the 

 length that is in the magnetic field. The end of the hollow tube that 

 enters the sound medium is generally sealed over with a fiat plate or 

 plug to increase the radiating surface and to provide a liquid seal 

 for the container. 



^^in- 



6 



B- 



6 



Rl 



o 



Fil 



Magnetic yoke 



O 



8+ 



6 O 



Magnetizing field 



Fig. 5. Schematic sectional view of a typical magnetostriction oscillator. 

 B — , B+, Fil, and C— are the electrical supply voltages. 



A schematic diagram of a typical magnetostriction oscillator is 

 shown in Figure 5. While more complex circuits have been used, 

 most biological or chemical investigations do not require them as long 

 as the required power and proper measuring devices are available. 

 The frequency of oscillation of the simple electron tube oscillator 

 pictured is determined by the relationship/ = l/[27r(LC)'^'] where 

 L is the inductance of the coil shown with the magnetostrictive tube 

 present and C is the capacitance. By var3dng C the frequency of the 

 oscillator can be adjusted to the natural period of the rod, at which 

 point the largest amplitude of vibration will be produced. Li is the 

 d.c. coil necessary to produce the proper static operating field for the 



