244 



Subsurface Geologic Methods 



inches inside diameter by 6^ inches with a one-fourth-inch wall) and sup- 

 ports a cylindrical cover of solid nickel half an inch thick, placed on the 

 block to shield the samples from direct radiation. Two complete units of 

 sample holder and thermocouples were prepared. Thus, if a break occurs 

 in one thermocouple, the entire unit may be recovered without delay and 

 a replacement connected. The next run may thus be carried out without 

 loss of time for repairs. 



The program controller is a special Leeds and Northrup "Micro- 

 max," which is connected to one of the four possible temperature-record- 

 ing thermocouples by way of a rotary selector switch. This unit is rated 

 to raise or lower the temperature of the sample at any desired rate from 

 0° to 50° C. a minute. It will also automatically hold the samples at 



1.5 V 



20ca 



-wm- 



o.4n 



-ww- 



J. *00Q 



-vvvvv-r^wv^T"Vvvy^T-vvw-r-vww-r^w^'-^-^^^ 



b 



•</6 MV* M/S MV« M/B MV« •t/6 MV« 



ifl 



. 10 Record 



.to Thsrmocoupit 



Figure 102. Potentiometer circuit for spreading records. The 

 unit is placed in series with one head of each thermo- 

 couple. The desired position for each couple is achieved 

 by connecting across appropriate terminals from a to g. In 

 the diagram, connection on a and b would add a con- 

 stant 1/6 mv. to the base line of the differential thermal 



any desired temperature when that temperature is reached. The pen record 

 indicates the temperature of the sample. The controller, when properly 

 adjusted, gives a linear heating curve. 



The recorder for the differential thermocouples is a Leeds and North- 

 rup "Speedomax," a six-point, high-speed, high-sensitivity electronic re- 

 corder with a maximum range of three millivolts. The chart of this re- 

 corder is synchronized with the chart containing the temperature record 

 on the program controller. This recorder is sensitive to 0.1° C. differ- 

 ential temperature, which, with the present specimen holder, gives a peak 

 one centimeter in amplitude for the alpha-beta quartz change. Experi- 

 mentation on increasing sensitivity with accessory devices is in progress. 

 However, it should be pointed out that, beyond a certain limit of sensi- 

 tivity, thermal gradients, geometry, thermocouple defects, and other un- 

 known factors cause prohibitive irregularities in the base line. The pres- 



