The search card blocks all light from the light source except for a pattern of very small beams defining the code that is to be sought. Beyond the film is a photocell. As the film containing images of documents moves through the machine, some of the light that passes through the search card will pass through the film and reach the photocell where it generates a low voltage electrical current. But when opaque dots on the film coincide exactly with the pattern of light beams defined by the search card, all light is blocked and no light reaches the photocell. When no light reaches the photocell, the flow of low voltage electricity falters.
Circuitry detects the loss of current and signals that the desired document has been found.
A modified movie projector was used, with suitable lenses to focus the beams of light on to the film, as shown in the lower half of Figure 2. Another mechanism was needed to project the image of the selected document for viewing or copying as shown in the upper half of Figure 2. In August 1931 the 8th International Congress of Photography was held in Dresden and Goldberg described and demonstrated the Statistical Machine at one of the technical sessions. It might have received more attention if the delegates had not been preoccupied by Goldberg's presentation of the proposal, eventually adopted, for an international standard for film speeds and, indeed, by his widely reported demonstration of sound movie technology. Goldberg himself clearly attached great significance to the Statistical Machine.