X. ULTRASONIC VIBRATIONS 



321 



of the energy that previously went into the mounting will now be re- 

 flected back and out the top face of the crystal. 



As a general rule, most quartz crystals are driven under oil be- 

 cause of its electrical insulating properties. If any other material is 

 to be irradiated, it is usually introduced into the sound beam in the 

 oil by means of a thin-bottomed beaker as shown. Salisbury and 

 Porter (6) have estimated that about one-third the energy radiated 

 into the oil bath can be transmitted to a liquid inside the glass vessel. 

 In case the crystal becomes too warm under continuous operation, 



(a) 



id) 



(c) (d) 



Fig. 7. Quartz crystal mountings as used in chemical and biological research. 



the oil may be circulated and cooled by external means. In c, a 

 bottomless beaker is cemented to the crystal and the one crystal 

 face drives directly the material in the beaker. Connection can be 

 made directly to the upper face of the crystal (if it is plated) or to the 

 liquid if it is a conductor. In this manner, intermediate media and 

 beakers are avoided; however, the insulation and breakdown prob- 

 lems remain. 



In d, the material under observation is placed on a crystal and ob- 

 served through a microscope. The crystal plating may be opened 

 at the point of observation to permit the passage of Ught if desired. 

 In this case, the crystal cannot be driven too hard because of possible 

 shattering and no acoustic measurements of the sound field can be 

 made. Without the latter, the observations become very empirical. 



