imbedded in the heat spreader electrical current is controllably driven through the resistive layer or the heating element so as to provide additional heat that must be dissipated by the cooling system thus in effect raising the forcing temperature of the device by a given amount this resistive arrangement serves for finely and accurately controlling the device temperature in a closed loop manner similar to that effected by the tec in the previously described configuration fig6 illustrates another embodiment of the invention wherein an extreme low device temperature is achievable by employing a cascading two stage refrigeration cycle in the schematic illustration of the refrigeration cycle the first stage refrigerant flows in closed loop conduit circuit 62 and second stage refrigerant which may be of a different type than the first stage refrigerant to provide an increased cooling rate flows in closed loop conduit circuit 72 the temperature of the first stage refrigerant may range from 0 to u2212 60 u00b0 c and the temperature of the second stage refrigerant may range from 0 to u2212 70 u00b0 c while their pressure may range from 0 7 to 24 bar the higher pressure levels are sufficient to maintain the refrigerant in a liquid phase the structure of the central unit is similar to that of the single stage cycle although provided with an additional compressor conduit circuit and heat exchanger and therefore need not be described for brevity the first stage refrigerant is pressurized by first compressor 64 to a high pressure p 1 and a high temperature t 1 and is then cooled by.