EDITOR'S COLUMN Continuous recording of the balance indication showed t h a t as the vertical gradient approached zero, instabilities commenced which increased as the bottom became warmer t h a n the top. As the t o p was allowed to warm, these instabilities decreased, a n d stability was reached when t h e t o p was once again w a n n e r t h a n t h e base. Again, a condition in which the bottom was but slightly (0.4° C.) warmer than the t o p resulted in rest point changes sufficient to prevent precision weighing. A test was then devised t h a t would show the damping effects of various types a n d configurations of insulation on t h e disturbance caused by a " s t a n d a r d heat pulse." H e a t was supplied by light bulbs placed 6 in. from one end of the balance. Of the m a n y insulation schemes tried, t h a t with approximately Vu-inch cork on the t o p a n d down both sides, b u t only halfway down the front and back, proved most successful. Such a configuration is used in the N B S Mass L a b oratory because of its marked a t tenuation of thermal disturbances. This arrangement of insulation not only provides ready viewing of t h e pans a n d pointer, b u t also permits heat to flow in from the observer and to warm some of the air, which rises to the top and helps maintain the desired thermal condition. Total insulation of t h e case has the undesirable effect of bringing the balance to an isothermal condition, which is less stable t h a n t h a t with air a t the t o p being warmer. Obviously, such major heat sources as the bulbs placed in some balances to illuminate the scale should, if possible, be removed, or a lower wattage bulb substituted. Also, it m a y be necessary to shield the balance from unequal radiation coming from t h e walls. An air conditioning system in releasing large quantities of cold air can create undesired thermal gradients within the case. T h e best environment is probably one whose temperature is rising slowly, with t h e balance temperature lagging slightly behind.
REXYN ION EXCHANGE
RESINS
IN THREE PURITY
GRADES
There are 40 Fisher Rexyn* ion exchange resins, including organic and inorganic resins in strong and weak acid and base, and mixed acid-base forms. Take your choice of these three purity grades: 1. Research Grade Rexyn provides extremely high purity and regeneration efficiency required in ultrasensitive analytical technique and research. Maximum metallic impurities: Fe, 0 . 0 0 5 % ; Cu, 0.0025%; Pb, 0.0025%; Ni, 0.0025%. 2. Chromatographic Grade Rexyn, designed for ion exchange chromatography, is available in two particle sizes (100-200 and 200-400 mesh); gives excellent resolution and sharp, clear separations. Maximum metallic impurities: Fe, 0.05%; Cu, 0.005%; Pb, 0 . 0 0 1 % ; Ni, 0.002%. 3. Analytical Grade Rexyn is economical, ideal for routine analytical use and pilot plant operations. Lot analysis of key chemical and physical properties on each label. For reference chart listing all Rexyn ion exchange resins, write Fisher Scientific Company, 100 J-190 Fisher Building, Pittsburgh 19, Pa. *Fisher Scientific Company Trademark
FISHER SCIENTIFIC World's Largest Manufacturer-Distributor of Laboratory Appliances & Reagent Chemicals Complete slocks in these locations: Atlanta · Boston · Chicago · Fort Worth . Houston . New York Philadelphia · Pittsburgh · St. Louis · Union, N. J. · Washington · Edmonton · Montreal · Toronto Circle No. 74 on Readers' Service Card VOL. 36, NO. 10, SEPTEMBER 1964 ·
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