outside air through a duct into the room. The fan for this also may be plugged in at O and operated thermostatically either instead of or in parallel with the fan in the cold closet. Lamp-cord connections are used at W and permit the relay to be located at any convenient place, since their length is immaterial. The air bath (fig. 26) on the polariscope mentioned on page 46 is located in this room. In operation the room temperature is set at from one-half to 11⁄2 degrees below that desired in the polariscope thermostat. The temperature of the latter is then brought up to and maintained at the desired temperature, usually 20° C, by an electric heater whose windings are fairly uniformly distributed around four surfaces of the box, as indicated in the photograph. A slight increase FIGURE 27.-Wiring diagram for electronic relay control of polariscope air bath (fig. 26). T1, 110- to 220-volt transformer, about 100 watts; T2, Filament-lighting transformer, 110 to 5 volts, 4.5 amperes; T3, Biasing transformer, 110 to 10 volts, 0.1 ampere. Note that T2 and T3 may conveniently be separate windings on T, VT, Thyratron FG 57; R1, 50,000-ohm, 1-watt grid resistor; R2, 1-megohm, 1-watt resistor: Ra, 200-ohm potentiometer (not needed if T3 is supplied with a mid-tap); L, Lamp sockets for the insertion of different-wattage lamps as series resistance for H; H, Heater-winding in thermostat; MC, Mercury-platinum contact contained in 1-mm capillary tube connected to toluene coil; and W, No. 20 lamp-cord leads to mercury contact. in temperature causes toluene in the glass tubing, fig. 26, to expand and close the mercury contact, MC, figure 27. This figure shows the wiring diagram for the relay and heater. The transformer, T3, working through the resistors RR2R3, when MC is closed, impresses a negative bias on the grid of the Thyratron, VT, sufficient to block off the plate current. When MC opens, this negative bias is removed and plate current again passes, since the grid potential then becomes 0 or positive, according to the position of the tap on R3. If a 10-volt middletapped transformer is available, R3 is not necessary. The heater, H, and the mercury contact may be seen in the photograph. The lamps, L, are outside the thermostat and serve to control the current in H without the necessity of varying the input voltage to T. Ordinarily a single 50-watt lamp gives about the right amount of series resistance, since only a few watts are required in H when the surrounding temperature is only about 1° below that of the thermostat. The air in the thermostat is stirred by means of a fan driven by a small electric motor. The temperature remains constant to 0.01° C or better. No variation can be seen on a 0.1° thermometer read by a telescope, where 0.01° C would readily be detected. There are no moving contacts to give trouble, and this relay has been left in operation for months at a time with no attention whatever. The current made and broken at the Hg-Pt contact, MC, has been reduced to a very small value, which largely eliminates contamination of the mercury due to arcing or sparking. If a screen-grid Thyratron, such as the FG 95, is used in place of the FG 57, the resistance in series with the contacts, MC, may be increased to several megohms without loss of reliability, still further reducing the current in MC. Recently a third constant-temperature room has been installed, designed to permit polariscopic measurement over a wide range of temperatures. The room is insulated by a 6-inch layer of cork on all walls, ceiling, and floor; the inner surface is finished in cement plaster on the walls, and thick cement floor. In the attic above is located a large insulated brine tank, cooled by immersed coils connected to the central system of liquid CO2 refrigeration. The cold brine is circulated through cooling coils mounted in the constanttemperature room by means of an electrically driven pump. The circulating pump may be run intermittently by a thermostat or continuously as desired. The cooling coil in the room is mounted in a socalled cooler placed near the ceiling. A large fan located in the cooler forces air over the coils and distributes it through a duct running the length of the room. The duct is fitted with suitable adjustable openings, permitting even distribution of the cooled air. A thermoregulator controls the pump which circulates the brine through the coolers. Other thermostats control heaters mounted within the duct. Constant temperatures may be maintained within the room over a temperature range of from -30° to +30° C. 5. REFERENCES [1] O. Schönrock, Z. Ver. deut. Zuckerind. 54, 521 (1904). VI. ACCESSORY APPARATUS 1. POLARISCOPE TUBES * (a) TYPES 89.1 (1939). (1) ORDINARY.-There are available at present a variety of polariscope tubes each designed to give satisfactory results under certain conditions. In making a selection, the user must be guided by his own particular needs. In general, polariscope tubes should be constructed to meet three essential requirements, and if these are not met by the device as furnished by the maker, the faults cannot be remedied by the user. These requirements are (1) parallel ends which are perpendicular to the axis of the tube, (2) accurate length, and (3) accurate centering of the axis of the tube in the trough of the saccharimeter. In most instruments the center of the optical path is approximately 15 mm above the bottom of the trough. The type for ordinary See test-fee schedule 297, p. 555. polarizations at temperatures not far removed from 20°C is the simplest, but, unfortunately, has received least consideration. Its design should have careful study, even to the slightest detail. This is essential owing to its widespread and constant use for both scientific and commercial purposes. An all-metal tube of small bore (fig. 28, No. 4), being one type that is in more or less general use, has the same diameter for the entire length. These tubes frequently are constructed with walls so thin that they are easily bent or distorted, resulting in a change of length and ruining the tube. In many cases the bore is too small. The diaphragming in the modern saccharimeter is designed to give the highest possible illumination of the field. To utilize this as well as to eliminate the undesirable "halo" in the field of the instrument, it is necessary that the bore have a diameter of not less than 9 mm. Considerable time is lost in filling owing to the fact that the tube must be so completely filled that no air bubble remains. The weight of the tube is carried upon the caps which hold the cover glasses. If the tube is rotated in the trough of the instrument, the caps may be tightened, and the additional pressure may cause double refraction in the cover glasses, which has the effect of changing the rotation of the plane of polarization. Glass tubes must be used when the solutions contain acid or other corrosive chemicals. Metal collars threaded to fit the screw caps are cemented on. Wax is sometimes used for this cementing, but this is objectionable on the ground that the wax sometimes softens and permits the collar to be displaced until it extends over the end of the tube. Thus the length of the column of liquid being polarized is increased and an error introduced in the observation. A mixture of glycerine and litharge or a similar cement is more satisfactory, and the ends of the glass tube should not extend more than 1 mm beyond the threaded collar. (2) BATES. In the laboratories of the U. S. Customs Service, as well as in other laboratories, there is required a tube which is both rugged and as free as possible of defects. The Bates tube (fig. 28, No. 1) designed at this Bureau, has proved to be entirely satisfactory in meeting these requirements. It will be observed that the weight is carried upon two shoulders, which are integral parts of the tube, and not upon the caps, thereby eliminating all danger of tightening when the tube is rotated in the trough of the instrument. The bore is 9 mm, permitting the utilization of the full aperture of the polarizing system. This also reduces to a minimum the light depolarized by reflection from the walls of the tube. The field of the instrument appears as a bright circle with no overlying haziness, and permits readings of increased accuracy. Both ends are enlarged with all the attendant advantages; hence but one size cover glass and washer is required. The walls are unusually heavy, eliminating all danger of bending. These tubes are available in 400-, 200-, and 100-mm lengths. Glass tubes of the same design are also in general use. (3) SPECIAL. There are a number of tubes available for special purposes. For polarizations, where the temperature must be controlled or measured, a water-jacketed tube is recommended (fig. 28, No. 2). This consists of an inner tube of either metal or glass having a tubulature midway between the ends to permit filling and inserting a thermometer. A watertight jacket of metal surrounds the inner tube provided with a nipple at each end to allow the circulation of hot or cold water. Landolt has designed a glass tube with one end enlarged and having sliding caps which fit over metal mounts, this construction being intended to eliminate the possibility of excessive pressure on the cover glasses. This tube has been modified by having the caps held in place by bayonet catches. However, the Landolt tube has not been generally used in the United States. Tubes provided with screw caps are preferred by most chemists, and if care is taken not to tighten the caps too much, they are entirely satisfactory. The continuous-flow tube of Pellet (fig. 28, No. 3) is widely used in factory control work. It is provided with a tubulature at each end, permitting filling and emptying without removing the tube from the instrument. The tube is filled by pouring the solution through a small funnel into one of the tubulatures. After the polarization, the succeeding solution is poured into the tube, displacing the first solution. These tubes effect a saving of time and are satisfactory for use with solutions of approximately the same polarization. Yoder devised a volumetric tube having the graduation mark on the connec tion joined on the middle of the tube. The usual size is 10 ml, but tubes of various volumes may be constructed by varying the length and bore. When the temperature of the solution is below the dew point, moisture condenses on the cover glasses. Wiley has overcome this by an ingenious desiccating cap (fig. 28, No. 6), which carries calcium chloride or other desiccant, and screws to the end of the polariscope tube. A special glass-lined Bates tube is shown in figure 28, No. 5. It is made in 50- and 25-mm lengths and is useful in cases where a limited volume of liquid is available or where high rotating liquids are to be measured. (4) WATER-JACKETED FOR HIGH-TEMPERATURE POLARIZATION.It is frequently necessary to make polariscopic observations at high temperatures. Difficulty has been experienced in such measurements, using the ordinary water-jacketed tubes, due to leakage and distortion of the field caused by uneven heating. There has recently been developed at this Bureau a tube designed to eliminate the existing defects. The new tube, figure 29, is constructed entirely of an ironnickel alloy having practically zero expansion for temperatures from FIGURE 29.-High-temperature polariscope tube (NBS type). 0° to 100° C. The tube is welded into a water jacket which extends well beyond the end of the inner tube, thus maintaining an even temperature throughout the length of the liquid-column under observation. The tube is provided with a tubulature for the insertion. of a thermometer. (b) TEMPERATURE CORRECTIONS It is customary to determine the exact length of polariscope observation tubes at the standard temperature of 20° C. The length of any tube at any temperature may be obtained by the following formula: L=L20 [1+B (t-20° C)], (35) where L20 is the length at 20° C and B is the coefficient of linear expansion of the material of which the tube is made. For glass, B0.000008, and for brass, 8=0.000019. From eq 35 for L20-200, L30 200.016 for glass and 200.038 for brass. It is evident that the errors resulting from changes in length of tubes of either material are negligible in ordinary use, but it is customary to apply the correction in all precision measurements. = (c) ACCURACY All types of polariscope tubes are accepted by the Bureau for test. The tolerances adopted by the National Bureau of Standards for polariscope tubes are given in table 8. |