ACCUR~TE weighing is particularly important in microanalysis, Lvhere sample weights are in the inilligrani range. One factor v h i c h m a y seriously alter the rest point of a microcheinical balance is a change in relative humidity. H. G. Xliiier of the S a t i o n a l Bureau of Standards has found t h a t enough moisture can collect on balance parts to move the pointer off scale. T h e effect i u minimized by careful design of coinponents, use of inaterials which absorb little moisture, and maxiniuni cleanliness. Four balances of different design were studied ; two \yere two-pan. equa 1- ar 11-1, and t w-o TTer e one - p a n , direct-reading type. Humidity was determined b y use of a n electronic hygrometer. After balance performance had been checked a t room temperature. moist air was introduced into the case. The balance indication \vas recorded while moisture escaped froin the case and the air returned to ambient conditions. When large changes in rest point were noted, the balance was cleaned and any lacquer and excess cement were removed. T h e test was then repeated. D r . Miner used a closed chamber technique t o determine the sensitivity t o humidity of various parts of the balance. W i i l e the effects of humidity change were different for each balance, each one reached a neIy rest point quite rapidly. Cleaning and removal of lacquer and excess cement improved performance. The order of change in the rest point before and after treatinent was on the order of 8.0 and 0.8 pg., respectively. T h e experiments showed t h a t the response of a balance t o changes in relative humidity is unpredictable. Moisture can collect on a balance
through absorption by hygroscopic material such a s lacquer, cement, and dust and by absorption on all surfaces. I n the design of balances consideration should be given t o differences in coefficient of adsorption and inequalities in surface area on either side of the beam. Scanning Photometer. S B S has developed a n improved scanning photometer t o determine wave lengths of spectral lines on a spectrographic photo easily, rapidly, and accurately. The instrument, developed by M. L. Kuder, optically scans a 03-mm.-wide portion of the plate. It presents the results, a curve of spectral density z's. w a r e length, on a n oscilloscope tube, After mounting the plate on an optical bench, any p a r t of the spectrum can be scanned manually by shifting the position of the moving carriage. T h e electronic equipment is stable and sensitive enough t o detect many lines in the hyperfine structure too faint t o be seen readily by the human eye. I n the NBS instrument, the plate is illuminated as one side. d 10power microscope on the other side is trained on a limited area of the spectrum. Behind the microscope is a dichroic mirror which splits the image into two coniponents-redyellow and blue-green. The red-yellolv image is projected onto a ground-glass vielying screen. The blue-green image is transmitted to a vibrating mirror which reflects a n oscillating image through a narrow slit t o a n end-on multiplier phototube. Although a total width of 0.5 micron is being scanned, the phototube sees only a 13-micronwide part of the spectrum. Five phototubes are used, each one detecting a separate spectrum including two standards. The displacent can be converted into a digital value of wave length and recorded on a punched card. The d a t a can then be analyzed or printed out b y a high-speed computer. T h e next step will be automatic reading of spectrographic plates. This will require equipment t o locate lines and measure their densities electronically. 85A
