X. ULTRASONIC VIBRATIONS 



315 



material. The actual relative change in length obtained in operation 

 is determined not only by the material used but also by the method of 

 clamping the rod and by the acoustic load presented to the rod. Ex- 

 amination of the graph shows that for nickel the addition of a pre- 

 magnetizing field on the order of 50 oersteds will not only prevent 

 frequency doubling, but will also place the operating point of the 

 rod at a position where the relative change in length is faii'ly large 

 for a given change in magnetic field. This point is rather critical 



Fig. 4. Relative change in length 

 as function of magnetic field 

 strength for various magnetostric- 

 tive materials. Negative values 

 signify contraction; positive values 

 denote expansion with increasing 

 magnetic field. 



200 400 



MAGNETIC FIELD (W) , oersteds 



600 



for best operation of the rod. As a general rule, the rods are not 

 premagnetized to this point but rather placed in a special magnetic 

 yoke that will allow the superposition of both d.c. and a.c. fields. 

 It is important to note that the peak a.c. field cannot exceed the bias- 

 ing d.c. magnetic field without producing excessive distortion of the 

 sound emitted by the rod. Consider as a simple example a nickel 

 rod 10.55 cm. in length vibrating at 20 kilocycles with a biasing field 

 of 50 oersteds. If we assume that the superposed a.c. field has an 

 amplitude of 10 oersteds, this will produce, as seen in Figure 4, an 

 amplitude of vibration of the free bar of: 



A = (5 X 10-6) (10.55) = 0.53 X lO"" cm. 



