The test chamber has been made transparent to enable us to visually check its flow properties to determine if any aerodynamic modifications are necessary. Pressure transducers are located approximately one inch on either side of the filter to enable us to determine the pressure drop through the filter as well as the absolute pressure level, Pressures to be measured during this phase of testing will vary from 0,1 to 10 mm of mercury. We pres- ently have available a variety of pressure transducers that are being used in our hypersonic wind tunnels that will cover this range. Preliminary calibration of these transducers have indicated a probable accuracy of + 5 to 10 thousandths mm of mercury. The first series of tests will be run at constant mass flow, varying pressure and velocity with time, The initial test conditions will be mass flows from 10°? to 10°? lbs/sec and pressure altitudes from 90, 000 to 200, 000 feet. Later, a Roots vacuum pump can be attached to the test section and any combination of expected sampler velocities and altitudes up to 300, 000 feet can be simulated for extended periods of time. Chemical analysis of the evaporator wire and the filter will enable us to determine relative filter efficiencies. The first filters will be cup-shaped to cover the entire inside surface of the test chamber as shown in Figure 7. walls, This will enable us to determine if any of the injected particles are diffusing to the test chamber The filters will be examined in sections to determine whether the cup shape is necessary. Figure 8 shows the evaporator wire circuit and a copy of an oscillograph trace taken during the preliminary work with the evaporator wire. This simple circuit enables us to quickly bring and hold the wire to any given temperature by varying the current to the wire. The oscillograph record is a plot of light intensity measured by a photocell versus time. It indicates the attenuation of a background light by the particles coming off the evaporator wire and condensing on the test chamber walls. The plot clearly shows the effect of the capacitor in bringing the wire up to temperature quickly. It also indicates attenuation of the background light and the wire light by the material evaporating from the wire and condensing on the test chamber walls. Preliminary tests with the evaporator wire technique have established a relation between the temperature and the resistance of the wire, It is estimated that particles of 1 to 10 molecules in size can be produced by this method, depending on the evaporation rate. We have a theoretical relationship between particle size and wire temperature which we are going to try to substantiate with some experimental results. For example, we might use the technique of introducing slides into the particulate flow and then examining these slides on an electron microscope to see if we can determine particle size. in the preliminary tests, tungsten wire with a Cadmium coating, which can be readily detected in the filter materials, was used. Asfilter testing progresses, other more standard methods of particle injection may be used to produce larger particles in the submicron-size range. 148