318 * EARLEC. GREGG, JR. 



tube. At high frequencies, the magnetic yoke shown must be of 

 laminated electrical steel to minimize eddy current losses. In some 

 cases, L and Li are wound on the same yoke. The power developed 

 by most oscillators for these frequencies ranges from 500 to about 2000 

 watts. At these higher powers, care must be taken not to overheat 

 the nickel tube because of the eddy currents present even though the 

 tube is slit. Water cooling has been used in some cases. 



While it is possible to produce sound at frequencies other than 

 the natural period of the metal tube, the amplitude of vibration is 

 extremely low and recourse is generally made to tubes of different 

 lengths. If the frequency range desired is too large, different a.c. 

 coils should also be used since the efficiency of the electron tube os- 

 cillator at various frequencies depends upon the coil construction. 

 Salisbury and Porter {6) describe a very excellent magnetostriction 

 generator of this type adapted to chemical and biological investiga- 

 tions. A 9 kilocycle commercial unit is available (40). Magneto- 

 striction generators are used primarily because of their ruggedness, 

 simplicity, and ease of producing high power. Their main disad- 

 vantages are the limited frequency range, the great dependence of 

 frequency on tube temperature, and the breadth of the natural reso- 

 nance curve. The last factor results from the change in elastic con- 

 stants of the metal tube with magnetization. The effect of clamping 

 and acoustic loading on the resonance curve will be discussed later. 

 To produce high sound energies at frequencies above 50 kilocycles, 

 recourse is generally made to piezoelectric crystals, but a 100 kilocycle 

 magnetostriction apparatus has recently been described {3d). 



2. Crystal Apparatus 



Ultrasonic vibrations may also be generated by the production of 

 mechanical strains in certain crystals when electrical charges are 

 placed on the proper crystal faces (4). From the viewpoint of elec- 

 trical, mechanical, and chemical properties, the best crystal today is 

 quartz, although tourmaline, Rochelle salt, and ammonium dihydro- 

 gen phosphate have been used. The two latter crystals, however, 

 because of thermal and cavitation effects at high amplitudes, are not 

 generally useful in chemical or biological work. Tourmaline would 

 be satisfactory except that it is not available in large crystals. 



There are many ways to cut a quartz crystal and still have it ac- 

 tive electrically. However, most of these cuts have special proper- 

 ties that are of little advantage in producing high energy sound. 



