296
INDUSTRIAL .4ND ENGINEERIYG CHEMISTRY
FIGURE 6. R H E O S T .4S3EMBl,IES ~P
inch). Rubber and fabric construction (Eastman Kodak Co., Rochester, N. Y., No. 57012 mechanism drive belt for Kodascope E or EE). This belt is a replacement part on motion picture projectors. ELECTRICAL CONNECTOR (Figures 5E, and 6). Midget cord connector with cord grip (Harvey Hubbell, Inc., 1930 Thomas St., Bridgeport, Conn., catalog No. 7478. Obtained from local electrical supply shop). The male end is connected to the motor leads which are clamped to the bodv (Figure 5 ) and the female end to the rheostat with 150 to 180-cm. ( 5 to 6 feet) of rubbercovered lamp cord. SUPPORTROD (Figure 50). The motor block is drilled and tapped (0.25-inch pipe size) at the center to accommodate the 18-8
VOL. 12, NO. 5
stainless-iteel alloy rod, 1.27-cm. (0.5inch) diameter, 30 cm. (12 inches) long. CLAMP(Figure 5 , F ) . Heavy-duty right-angle clamp fastener. By using a large swivel clamp fastener, motion in the third plane can be obtained if desired (Central ScientificCo., 1700 Irving Park Blvd., Chicago, Ill., Catalog No. 72315). RHEOSTAT(Figure 6). A 5(rwatt, 400-ohm rheostat, with or without an off position in the extreme counterclockwise position according to the mounting shown, has proved satisfactory (Hardwick, Hindle, Inc., Newark, N. J., Type B-50). Previously, lobwatt rheostats (Ohmite Manufacturing Co., 4835 West Flournoy St., Chicago, Ill., Model K, 4O0-ohm1 stock No. 0454) had been used for 3 years. The smaller unit, however, costs only half as much and has shown no deterioration over a period of 6 months, though used above the manufacturer's current rating. The rheostat is mounted between two pieces of hard asbestos sheet 0.3 cm. (0.125 inch) thick, and a 8.9-cm. (3.5-inch) dial plate calibrated 0" to 100' over 325" is used to indicate the setting. This may be purchased from a local radio supply store. It is connected to the power supply through 60 to 90 cm. (2 to 3 feet) of lamp cord and a male electrical plug.
Literature Cited \ I ) Hershberg, IND.ENQ.CHEM.,-4nsl. Ed., 8 , 313 (1936).
Thermometer for Low Temperatures A. FARKAS AND L. FARKAS, Department of Physical Chemistry, The Hebrew University, Jerusalem, Palestine
HILE t h e well-known vapor pressure thermometers of Stock are the most accurate instruments for measuring low temperatures, they appear unhandy if the measurements are to be performed in a limited space. T h e v a p o r pressure thermometer having the form shown in the accompanying figure eliminates this drawback, and has proved very satisfactory in use in this laboratory. The construction of parts A , B , and C is evident from the figure. Coil D has a small hole in its end which prevents the mercury in B from entering tube C, while it ensures communication between Bland C. Tube E is sealed to the short glass rod, F , which in turn is sealed inside tube A . The capillary tip, G, of tube E prevents the escape of mercury from E in case the thermometer is kept in the horizontal position. The instrument is filled as follows: Bfter adding sufficient mer-
cury, t h e thermometer is brought into horizontal position with coil D upward (the mercury should now reach the level indicated by the dotted line), and is evacuated. Then the thermometer is brought into perpendicular position, the thermometric substance-e. g., carbon dioxide-is admitted up to a pressure of 400 to 500 mm. and condensed, and the thermometer is sealed off a t a. The thermometer can be calibrated empirically or by using the known vapor pressures of the thermometric agents. In the latter case the capillary depression of the mercury in tube E has to be allowed for. One thermometer of the dimensions given in the figure will cover about 15" to 2 5 O , depending on the gas with which it is filled. Ten thermometers filled with the substances listed in the table will cover nearly every range of temperature down to -215" C. Once a thermometer has been filled and sealed, i t can be handled just like any ordinary mercury thermometer. Thus it will not be affected if it is kept in the horizontal position. Substance
Temperature Range O
Pentane Butane Propane Carbon dioxide Ethane
c.
+ 5 to - 35 to -
35 73 71 t o -100 - 93 to -112 -112 to -140
-
Substance
Temperature Range
c. Ethylene Nitric oxide Methane Oxygen Nitrogen
-122 -159 -175 -193 -205
t o -150
to -175 t o - 188 to -205 to -215
