Figure 3-4. Timer (S) circuit.
e. The photoelectric timer requires the use of a photocell or photoelectric eye. A
small fluorescent screen with a photocell is placed behind the cassette (a film holder,
having a back through which x-rays may penetrate). When a predetermined quantity of
radiation has struck the fluorescent screen, causing it to give off light that the phototube
measures, a mechanism is activated which automatically stops the exposure at the
correct time. This automatic timing device produces radiographs of exactly the same
density. Photoelectric timers are used extensively in photofluorography, and have been
found to be very useful in connection with spot film exposures in gastrointestinal work.
However, they are not yet widely used in making conventional diagnostic radiographs.
Following is a discussion of figure 3-5 which shows the entire circuit layout for a
full-wave rectified x-ray machine.
a. Device number one marks the on-off switch. When this is closed, a complete
circuit is formed with the incoming line. The incoming line-voltage passes through the
double line fuses (device number two) and then on to both the internal and external line-