Mind, Body, World- Foundations of Cognitive Science, 2013a

2.5 Relays and Multiple Realizations
Many of the ideas that we have been considering in this chapter have stemmed from
Shannon’s (1938) logical interpretation of relay circuits. But what is a relay?
A relay is essentially a remote-controlled switch that involves two separate circuits
(Gurevich, 2006). One of these circuits involves a source of current, which can
be output through the relay’s drain. The second circuit controls the relay’s gate. In
an electromechanical relay, the gate is an electromagnet (Figure 2-2). When a signal
flows through the gate, the magnet becomes active and pulls a switch closed so that
the source flows through the drain. When the gate’s signal is turned off, a spring
pulls the switch open, breaking the first circuit, and preventing the source from
flowing through the drain.
Source
Drain
Gate
Figure 2-2. A relay, in which a signal through an electromagnetic gate controls
a switch that determines whether the current from the source will flow
through the drain.
The relay shown in Figure 2-2 can be easily reconfigured to convert it into a NOT
gate. This is accomplished by having the switch between the source and the drain
pulled open by the gate, and having it closed by a spring when the gate is not active.
This was how, in 1835, Joseph Henry turned the power off to a large electromagnet,
causing it to drop its load and startle his class (Moyer, 1997).
The type of relay shown in Figure 2-2 was critically important to the development
of the telegraph in the mid-nineteenth century. Telegraphs worked by sending
electrical pulses—dots and dashes—long distances over copper wire. As the signals
travelled, they weakened in intensity. In order to permit a message to be communicated
over a long distance, the signal would have to be re-amplified at various
points along its journey. Relays were the devices that accomplished this. The weak
incoming signals were still strong enough to activate a relay’s magnet. When this
happened, a stronger current—provided by the source—was sent along the telegraph
wire, which was connected to the relay’s drain. The relay mechanism ensured
that the pattern of pulses being sent along the drain matched the pattern of pulses
that turned the gate on and off. That is, the periods of time during which the relay’s
Multiple Levels of Investigation 35