VOL. 12, KO.12
INDUSTRIAL AND ENGIREERING CHEMISTRY
754
uniform and satisfactory source of illumination. A cheaper, but less satisfactory, light source is a row of "eight in series" white Mazda Christmas-tree lamps behind the milky glass diffusing screen. Particularly with the Christmas-tree lamps, and to a less extent Tvith the fluorescent lamp, some cooling is desirable to prevent the film, scale, etc., from becoming warm and expanding during the course of a series of measurements. Such cooling is achieved by bloiving compressed air through a series of holes in a brass tube, B , running the length of the case. Air holes in the back of the case permit circulation of the cooling air. C is an aluminum alloy (17ST) plate, 46.45 X 12.5 X 0.469 em. (18.5 X 5 X 0.188 inches), attached to the wooden case a t three points. I t has a 2.5 X 35 em. (1 X 14 inch) opening directly over an illuminated slot in the wooden case. A piece of plate glass with its upper surface flush with the surface of C fills this opening, and serves as a support for 2.5 X 32.5 em. (1 X 13 inch) powder films which are temporarily held in place by the spring clips D, a t each end. E is a similar aluminum alloy plate, 46.45 X 8.75 cm. (18.5 X 3.5 inches), hinged to C a t the upper edge. A 1.25 X 35 em. (0.5 X 14 inch) beveled slot in it is centered over the opening in C. E is lifted up for insertion of a film and returned to position. When the film has finally been adjusted to the exact position for measurement it is clamped tightly in position between plates C and E by the removable thumbscrews, 111,which are threaded into C. Plates C and E must be flat and have good surfaces. To ensure this it is well to specify that they be cut by sawing rather than by shearing. The scale, F , is a 60-em. vernier caliper (KO. 122 ?vl, L. S. Starrett Co., Athol, Mass.) graduated in 0.5-mm. divisions and provided with a vernier reading to 0.02 mm. I t is clamped in position 1.875 cm. (0.75 inch) above plate E by the slotted blocks, G, fastened to E. Clamped to the movable jaw is a 2.5-em. (1-inch) 4 X magnifying lens, in a tubular focusing mount, H , for readin the vernier. The vernier is illuminated by a 2.5-volt flashli&t bulb in tube J . One contact is made at the end of J by a flexible wire, K , and the other through the metal parts
of the instrument. A toggle switch on the left end of the case controls the light whose current source can be two dry cells or a small transformer. 9 rectangular lens would give a slightly better field in reading the vernier. The movable jaw was also drilled at L and provided Ivith setscrem for holding a pointer, P , in place. In Figure 1 is shown a detail of P , which is used in the horizontal position. The angle of the point should not be too sharp nor too broad-approximately 15' has been found satisfactory. The tip of the pointer should not end in too sharp a point but should be slightly dull. The pointer is adjusted to be just the thickness of a thin sheet of paper above the film, thereby preventing parallax errors. With this type of pointer and a low-power reading glass it is possible to set on the edge or center of a line x i t h ease and accuracy. For example, in determining the position of the undeviated beam image on a powder film a single setting was made on the center of each of five pairs of lines symmetrically situated xyith respect to the exit beam. These measurements gave for the position of the beam image 36.308, 36.308, 36.306, 36.305, and 36.307; average, 36.306. Rotation films are inserted in the device and shifted to bring successive layer lines into position for measuring.
Acknowledgment The author takes pleasure in acknowledging the helpful auggestions and excellent workmanship of Floyd 0. Grapp, instrument maker, in the construction of this instrument.
Literature Cited (1) Jette and Foote, J . Chem. Phys.,3,605 (1935).
( 2 ) Owen and Pickup, Proc.R o y . SOC.(London),137, 397 (1932). (3) Schlossberger, 2. Krist., 98, 259 (1937). (4) Straumanis and Mellis, 2. Physik,94, 184 (1936). (5) Tatel and Hultgren, Rea. Sci. Instruments, 9, 47 (1938).
A Centrifuge Cooling Unit H. L. WUNDERLY A X D L. s. SMELO Renziehausen Diabetic Foundation, Children's Hospital, Pittsburgh, Penna.
S
OMETIMES it is desirable to keep liquids 5" to 10" below room tem-
perature during centrifuging. For those lacking a refrigerating unit a n attachable cooling unit utilizing solid carbon dioxide may prove satisfactory, Such a unit was devised in this laboratory for the centrifuge (International Equipment Company, size 1, type SB centrifuge). The unit (Figure 1) consists of a metal container, 9 inches in diameter and 2.5 inches deep, within which is a short copper coil (gas stove copper connection) the open ends of which project through the centers of the top and bottom of the container. The coil is held in place by solder where it is inserted through the bottom of t,he container. The unit is packed with solid carbon dioxide by removing the lid. Then it is assembled and adjusted to t,he underside of the lid of the centrifuge by the threaded end of the copper coil, which projects through both the lid of the container and the lid of the centrifuge and is held firmly by means of a metal nut. Sufficient clearance must exist between the cooling unit and the head so that there is no possibility of the centrifuge tubes coming in contact x-ith the cooling unit. A metal brace to fit around the container gives support to the unit. If the lid of the centrifuge does not fit firmly on the protecting case when enclosed, insulating with adhesive tape adds to the efficiency of the cooling process.
The temperature attained in the centrifuge is dependent on the quantity of solid carbon dioxide placed in the container and the temperature of the laboratory. OF COOLIXG GXIT TABLE I. EFFECTIVENESS
(During 10-minute periods of centrifuging water initially cooled t o 20') F i n a l Temperature of Water F i t h cooling unit a t Without cooling unit a t room temperature of room temperature R. p. m. 28' C of 29' C.
FIGURE 1. COOLING UXIT Above. Below.
Adjusted t o centrifuge lid Disasaembled
1100 1800 2400 2800
28.0 29.0 30.0 32.0
20.5 20.0 21.5 21.5
