A complete setup showing the transmitting antenna at the left,
with the receiving antenna at right. Note the adjustment screw
on top of the receiving antenna, which is used to adjust the pressure
of the point-contact detector (see Fig. 5). In the center is a
rotating table (the "spectrometer circle" of Figure
2) on which various microwave components (prisms, lenses, grids,
polarizers etc.) may be mounted for study. A dual-prism attenuator
(see below) is shown in this photograph. The arrangement as shown
is not yet properly aligned.
Figure 5 shows two of Bose's point contact detectors. In use,
the detector would be placed inside an overmoded waveguide receiving
antenna, very much like the transmitting antenna shown in Figure
4, and with a matching polarizing grid.
the I-V characteristics of his junctions; an example characteristic
curve of a "Single Point Iron Receiver" is shown in
Figure 6. The junction consisted of a sharp point of iron,
pressing against an iron surface, with pressure capable of fine
adjustment. The different curves in Figure 6 correspond to different contact pressures.
Bose noted that the junction does not obey Ohm's law, and that
there is a knee in the curve at approximately 0.45 volts; the
junction becomes most effective at detection of short wavelength
radiation when the corresponding bias voltage is applied. He made
further measurements on a variety of junctions made of different
materials, classifying the different materials into positive or
Figure 5. Two of Bose's point contact detectors,
removed from the receiving antennas.
classes of substance. In one experiment he noted that increasing
the applied voltage to the junction actually decreased the resulting
current, implying a negative dynamic resistance .
Bose's short-wavelength detectors is the spiral-spring receiver.
A sketch of a receiver used for 5-mm radiation is shown in Figure
7; the spring pressure could be adjusted very finely
in order to attain
optimum sensitivity. The sensitive surface of the 5-mm receiver
was 1 by 2 cm. The device has been described recently  as a
"space-irradiated multi-contact semiconductor (using the
natural oxide of the springs)." A surviving, somewhat larger,
spiral spring receiver is shown in the photograph Figure
8. The springs are held in place by a sheet of glass,
seen to be partly broken in this example.