The operating characteristics of the apparatus depend on its exact construction, and must be determined in preliminary experiments. The following results indicate the performance to be expected. First, the flow of water n-as adjusted to a convenient rate, approximately 1 liter per minute. Secondly, a calibration curve was constructed by maintaining the reservoir a t various temperatures, and determining the temperature attained in the sample cells after steadystate conditions n ere established. As the data gave a straight line, fire measurements n ould in general be sufficient for delineating it. From the line, it could easily be determined what reservoir temperature would give any desired temperature in the cell compartment. Minor additional adjustments, if needed, were then made by changing the reservoir temperature further. In the apparatus used in this work,
sample-cell temperatures of 20°, 30°, 40°, and 50" C. required reservoir 29", 45.5", and temperatures of Eo, 62" C., respectively. When the reservoir temperature was set a t 0" by filling with ice and water, the temperature established and maintained in the sample cells was 12.5'; however, no regulation was possible in this case (when absorption measurements are done below room temperature. condensation of moisture on the faces of the cells must be guarded against; this was not serious a t 12.5"). When the instrument was originally a t its normal operating temperature, the time required before the sample cells reached the required temperatures was about 15 minutes for 20" and 30°, and as long as 40 minutes for 50". Once attained, the temperature could be maintained within k0.5' for 1 hour or longer. I t is not good practice to keep samples in the cell compartment
for a long time. If measurements are desired a t long intervals, it is preferable to keep the sample in a thermostat, and to transfer it to the cell compartment when the measurements must be taken. The parts of the instrument next to the cell compartment are heated only very slightly even when water from a reservoir a t 70" C. is passed through for some hours. The wave-length scale was checked with the mercury line a t 436 mp, and found to vary by less than 1 mp during this time. ACKNOWLEDGMENT
Heinz Hall and Frank Hargrove constructed the special apparatus parts described and their assistance is gratefully acknowledged. WORKsupported by the National Science Foundation through Grant G-5966 and the Research Participation Program, Oklahoma State University, summer 1959, of which Harold J. Martin was a participant.
X-Ray Diffractometer Gear-Changing Mechanism A. C. Lilly, 1. H. Milne, W. T. Caneer, and J. A. Dalzell, Gulf Research and Development Co., Harmarville, Pa.
E
operation of the Philips high angle diffractometer for semiquantitative analysis of mineral mixtures requires that the scanning speed be as fast as possible. However, a certain amount of resolution and intensity accuracy is sacrificed a t high scanning speeds and it is frequently necessary to record portions of the diffraction spectrum a t slower rates. Changes in scanning speed are normally accomplished by manually changing a pair of gears on the front of the goniometer. This procedure becomes timeconsuming when a large number of samples with differing requirements are involved. The problem has been solved in this laboratory by installation of a mechanical gear-changer which permits rapid selection of three different gear ratios. The details of the gear changing mechanisms are illustrated in a cross section of the apparatus shonm in Figure 1. FFICIENT
The driving shaft of the goniometer motor has been extended to support a movable sleeve upon which are mounted three gears increasing in diameter from front to back. The driven gear shaft has been extended also to carry three gears arranged in decreasing diameter from front to back. Spacing of gears on the shafts is arranged so that one pair of gears is engaged a t a time by forward or backward movement of the sleeve on the motor shaft. The sleeve is fixed to the motor shaft in a rotational sense by a slot arrangement. .4 knob
2.7083 0 D
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S C A L E IN INCHES
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Figure 1 .
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Layout drawing of gear-changing mechanism
Area between A and A' i s cross-sectioned to indicate internal construction. ratios. AA' 1 :4; BB' 1 : 1; CC' 4: 1
is available on the outside of the gear box to move the sleeve, and ball-ingroove stops are provided to locate correct positions. The gear-changing mechanism has been operated for a year with the goniometer motor running, without noticeable damage to the aluminum gears. However, if more expensive gears were used, it would be advisable to stop the
Gear
motor before changing gears so as to prolong gear life. Specific scanning rates may be selected on the basis of the gear ratios of the three pairs of gears which are installed. At present the mechanism is installed on two diffraction units in this laboratory, one of which provides scanning speeds of 4O, lo, and per minute and the per minute. other 2", 1/20, and VOL. 32, NO. 7, JUNE 1960
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