relies on a vacuum source not shown located outside of the environmental enclosure the vacuum source communicates through appropriate control valves and piping with apertures not shown in the upper surface 12 of the chuck 4 when the device under test 10 is placed on the chuck 4 the device blocks apertures leading to the vacuum source air pressure holds the device under test 10 against the chuck 39 s upper surface 12 one or more probes 6 can be positioned over the device under test 10 and brought into contact with test pads on the circuit to be tested instrumentation connected to the probes 6 measures selected operating parameters of the circuit at the test pads a thermal chuck 14 bracketed may be used to test the operation of devices at temperatures other than the ambient temperature of the environmental enclosure 2 referring to fig2 the thermal chuck 14 indicated with a bracket may include a thermal driver 16 having facilities for modifying the temperature of a chuck 4 indicated with a bracket supported on the top of the thermal driver 16 the thermal driver 16 may be arranged to provide for either heating cooling or heating and cooling of the chuck 4 the thermal driver 16 comprises one or more electrically powered thermal units 20 each of which includes one or more thermal devices 22 and a plurality of insulated power conductors 24 connecting the thermal devices 22 to a thermal driver controller 18 typically the thermal devices 22 are resistance heaters or thermoelectric heat pumps resistance heaters and thermoelectric heat pumps can increase the temperature of the chuck 4 the thermoelectric heat pump can also be used to cool the chuck 4 the thermoelectric heat pump.