A Technical description of the Handset
by Elsa Garmire
Drawing/diagram of Handset by Elsa Garmire. Figure F1
In the entrance tunnel, handsets are obtained. These clear plastic cylinders emit light and sound when held above certain floor areas such as the soft entrance of the Clam Room and most of the floor of the Dome Room.
The components of the handset are a small speaker in the top, battery below that, then circuit board, antenna coil, light bulb and recharging jack.
The antenna coil in the handset picks up electromagnetic signals produced by current loops in the floor. The pick-up mechanism is magnetic induction, similar to what produces "cross-talk" on telephone lines, those weak voices heard from neighboring wires. Induction is the mechanism by which a transformer works. When two wire coils are placed one inside the other, electric current fed into the primary coil is picked up and transmitted by the secondary coil with a voltage different from the primary voltage by the ratio of the number of turns in the two coils. In this case the floor loops can be considered the primary, and the handset is the secondary.
An amplifying circuit above the handset coil composed of 11 transistors and a diode, drives the speaker in the top end of the handset. The same pick-up coil and amplifier is also used to switch the light. This is arranged by preceding the light-switching transistor with a low-pass filter so that acoustic frequencies do not reach it (see Figure Fl). 60 cycles is used as the light-switching signal, a frequency below the response of the speaker.
The light and amplifying circuit are driven by a 6V rechargeable battery containing 500 milliamp hours. Current drains are such that the handset can be used for about fifteen minutes before fifteen minutes of recharging is required. The handsets will also be charged continuously each night when the Pavilion is closed. The recharging jack is located in the bottom, with the light bulb.
The large current loops embedded in the floor which drive the handsets were especially designed for the Pavilion. The floor of the Dome Room has eleven signal-emitting segments each with roughly 150-square-foot area. The handset must respond with the same strength all over a floor area and yet not pick up signals from one floor area while positioned above an adjacent floor area. These conditions, plus that of obtaining as much pick-up signal as possible, resulted in a system of hundred-turn coils a foot in diameter, spaced4.5 feet from each other in a lattice array. Each floor area, then, contains roughly twenty loops. Each loop area of the floor is wired to a tape recorder and power amplifier in the control room. A 60-cycle signal generator and relay add the light control function to the audio signal (see Fig. Fl).