MECIIAXICAL PROPftETIES OF POLYMERS 



131 



use of one crystal for both receiving and transmitting, the crystal is put 

 in the bridge circuit of Fig. 5 where a resistance and capacity are used 

 to bahuice out the transmitted pulse so that it will not overload the 

 amplifier. The relatively weak voltages generated by the incoming 

 acoustic waves pass through directly. The gate circuit provides pulses 

 of radio freciuencj^ voltage at repetition rates in the range of 20 to 100 

 per second with a synchronizing voltage supplied to the oscilloscope for 

 the horizontal sweep. The frequency range of the device is from 20 to 

 200 kc. Both glass and nickel-iron rods were used, the latter having a 

 very low frequency-temperature coefficient. With a 100-kc quartz 



REFERENCE 



BUFFER 

 AMPLIFER 



ATTENUATOR 



TO 

 AMPLIFIER 

 AND 

 OSCILLO- 

 SCOPE 



Fig. 5 — Experimental pulsing circuit for measuring torsional impedance of 

 liquids. 



torsional crystal, a rod length of 21 inches and diameter of 0.2 inch w^ere 

 used. The entire crystal-rod assembly is placed inside a glass tempera- 

 ture control unit, as shown by Fig. 4, through which water can be cir- 

 culated to provide temperatures in the range 0°C to 80°C. The test 

 liquid is placed either directly into the inner bore of this water jacket, 

 or in another tube which can be inserted from the bottom to surround 

 the rod up to a fixed mark. 



In use, both phase and attenuator settings were adjusted to balance 

 the first received pulse against the continuous wave component passing 

 through the attenuator. Cancellation for the duration of the pulse was 

 visuall}^ indicated on the oscilloscope. A plot of balance phase and level 

 is made as a function of the temperature. AVhen the liquid is introduced 

 an attenuation change AA and a phase change AjB are required to 



